Yesteryear (episode)
Spock travels back in time to prevent his own demise during his youth on Vulcan.

Summary
Spock is transported by the Guardian of Forever to his childhood to save his younger self from dying during a Vulcan adolescence ritual and spawning a new timeline in which he is not the 's first officer.

Act one
"Captain's log, stardate 5373.4. We are in orbit around the planet of the time vortex, the focus of all the timelines of our galaxy. Our mission is to assist a team of historians in the investigation of Federation history."

As Federation historians Aleek-Om and Grey as well as  Doctor McCoy record history, while standing outside the Guardian of Forever, the Guardian announces the return of the travelers Kirk, Spock and historian Erickson through its gateway, after a mission to observe the dawn of the  civilization. As the travelers begin to appear, one-by-one, Kirk announces to McCoy how exciting it was to observe the birth of a civilization, just as the last traveler, Spock, emerges from the Guardian, much to the surprise of both Grey and McCoy. McCoy inquires who it is they brought back with them. Although Kirk reacts with puzzlement because he believes McCoy obviously knows Spock, McCoy says he doesn't.

The crew members beam back to the ship and are greeted by Scotty. He was not expecting a Vulcan among those he transported back, and was anticipating one of the historians instead. Annoyed, Kirk responds that he expects his first officer to be treated with respect, just as an Andorian crew member enters the transporter room, stating that no-one has ever treated him otherwise. Kirk inquires as to who the newcomer is and McCoy then introduces – or rather re-introduces – Commander Thelin, making an attempt to remind Kirk that Thelin has been his first officer aboard the Enterprise for the past five years. Spock and Kirk finally come to the realization that what they have encountered is not a game, and question what is going on.

"Captain's log, supplemental. When we were in the time vortex, something appears to have changed the present as we know it. No one aboard recognizes Mr. Spock. The only answer is… that the past was somehow altered."

In a staff meeting in the briefing room, Lieutenant Erickson reviews the tricorder logs from their mission, and observes that there is nothing they could have possibly done to change the events of the future. Spock then surmises that the change in the timeline must have happened while they were in Orion's past.

The meeting is then interrupted by Bates, who just checked Starfleet records on Spock, as requested by Thelin. Bates reports there is no Vulcan named Spock serving with the Starfleet in any capacity. Thelin then inquires about the results of the Vulcan family history request. Bates displays an image of Sarek of , and notes that he has been an ambassador to seventeen Federation planets in the past thirty years. Spock notes that this information is incorrect and inquires about Sarek's family – his wife and son. Bates then transmits an image of Amanda Grayson, the former wife of Sarek, from whom she separated following the death of their son. Amanda was later killed in a shuttle accident at Lunaport on her way home to Earth. Sarek has not remarried. Spock mourns briefly for his mother, then inquires as to the name of the son that died; it is Spock, aged seven upon his demise.

Kirk, Spock, and Thelin return to the surface of the time planet to confer with Aleek-Om and Grey on what happened. Kirk asks if the Guardian was in use while they were away, and Grey informs him that it was used, in only in a limited capacity, to scan recent Vulcan history, twenty to thirty Vulcan years past. Kirk then wonders if there is any notation on the death of Sarek's son, and Aleek-Om confirms that there was, and that he had died during the kahs-wan maturity test.

Spock then recalls the date of the event in question, the twentieth day of Tasmeen, noting that it was the day that his cousin Selek saved his life in the desert when he was attacked by a wild animal. Although Spock cannot remember the finer details of the event, he recalls that that was also the only time he had ever met Selek.

On a hunch, Kirk asks if Selek looked anything like how Spock now does. Spock confirms what Kirk has been thinking; Spock saved his own life in the original timeline, but was unable to do it a second time because he was in Orion's past when the time vortex replayed Vulcan history, making it impossible for him to be in two places at one time. With the realization of what has transpired, Kirk asks the Guardian if there is any way they can reverse what happened. The Guardian confirms that there is, as long as no other important factor is altered.

Spock decides that, in order for him to save the lives of both his mother and himself, he has to return to Vulcan and correct what has been changed. Before his departure, he requests a Vulcan desert soft-suit and boots, as well as a small selection of street wear and carry bag, circa 8877 Vulcan year, from the ship's wardrobe section. Kirk flips open his communicator so it can be done. A brief conversation with Thelin reveals that he bodes no ill will towards Spock's quest, despite the fact that this quest will change the commander's timeline as well. He considers it a reasonable sacrifice in order to save Spock's family. Likewise, Spock wishes Thelin a long and prosperous life, in whatever circumstances the hopefully repaired timeline will put him in.

Spock hurriedly enters the Guardian and travels back to his hometown, ShiKahr, in the month of Tasmeen, thirty years in the past, in 2237.

Act two
"Personal log, stardate 5373.5, subjective time. I have returned to the past in an attempt to restore the future. I am home, and I have almost forgotten its beauty."

Spock meets his father, Sarek, as well as his younger self (who is being harassed by three young Vulcan bullies – Sepek, Sofek, and Stark), and assumes the identity of cousin Selek, introducing himself to Sarek in such a manner.

"Personal log, stardate 5373.9, subjective time. The timeline seems to have changed again, yet I do not believe I have done anything to disrupt it. My memory is quite clear regarding the date my cousin saved my life and it is tomorrow. The kahs-wan ordeal is an ancient rite of warrior days. When Vulcans turned to logic, they reasoned they must maintain the tests of courage and strength to keep pure logic from making them weak and helpless."

Young Spock goes on his kahs-wan a month early, on his own will. His pet sehlat, I-Chaya, follows him into the Vulcan desert, defying young Spock's authority. In the meantime, adult Spock/Selek, about to turn in for the night, suddenly realizes and says aloud, "Of course– I should have remembered! It wasn't the actual kahs-wan ordeal." Upon that realization, he rises again and dresses, following young Spock and I-Chaya into the desert.

"Personal log. The boy, Spock, should be moving towards the L-langon Mountains. He – I, had much to prove to myself. The personal ordeal upon which I embarked was meant to determine the course my life would take."

In the desert, the pair are attacked by a ferocious le-matya. Adult Spock, who had been following the two into the desert, intervenes and saves young Spock's life, incapacitating the large creature with a Vulcan nerve pinch – but he is too late. For unlike the way adult Spock remembers this episode, I-Chaya is badly injured from the battle and is dying.

Act three
Young Spock sets out to find a healer in the city. Meanwhile, adult Spock uses a nerve pinch to help ease I-Chaya's pain, telling the creature that he previously undertook the kahs-wan ritual without I-Chaya having to be sacrificed.

Although young Spock is successful, I-Chaya is too far gone by the time the healer arrives. The healer gives young Spock the choice to give I-Chaya a longer but painful life, or to release him from his suffering. Spock chooses to release I-Chaya from life, the most logical way, and, in doing so, chooses to follow the Vulcan ways of his father.

The timeline is saved and Spock returns from the Guardian as first officer. Before beaming back to the Enterprise, Spock laments the death of I-Chaya to Kirk, admitting that this did not happen the first time around. Kirk, however, cannot understand how a pet could mean so much in the course of time. In the Enterprises transporter room, McCoy is complaining when it is Spock's turn to take a routine physical, and bemoans that he has to recalibrate his medical scanner for a Vulcan every time. Spock tells him that if things were different, he would be recalibrating for an Andorian instead. McCoy asks if this is some sort of joke, reminding Spock that Vulcans don't tell jokes. To this, Spock responds by pointing out to the doctor that "times change."

 Log entries 
 Captain's log, USS Enterprise (NCC-1701), 2269
 Personal log, Spock

Memorable quotes
"Is it possible for Spock to return to Vulcan and repair the timeline that has been broken so all is the same as before?"
"It is possible if no other major factor is changed."
 - Kirk and The Guardian

"What a trip, Bones!"
 - Kirk, to McCoy after emerging from the Guardian of Forever

"In the family, all is silence. No more will be said of it. Live long and prosper, Sarek of Vulcan."
 - Spock (as Selek) to Sarek

"Who are you?"
"Oh, I thought sure you'd know Thelin by now, Jim. He's been your first officer for five years."
 - Kirk and McCoy

"You will not disappoint me. Not if your heart and spirit are Vulcan."
 - Sarek, to young Spock on his upcoming kahs-wan ritual

"What you do not yet understand, Spock, is that Vulcans do not lack emotion. It is only that ours is controlled. Logic offers a serenity Humans seldom experience in full. We have emotions, but we deal with them... and do not let them control us."
 - Spock, to his younger self

"Earther! Barbarian! Emotional Earther! You're a Terran, Spock! You could never be a true Vulcan!"
"That is not true! My father..."
"Your father brought shame to Vulcan. He married a Human! You haven't even mastered a simple Vulcan neck pinch yet, Earther!"
 - Sepek, Stark, and Spock

"What's the matter, Bones?"
"Who's he, Jim?"
"Whadda you mean, 'Who's he'? You know Mr. Spock."
" 'Fraid I don't, Jim."
 - Kirk and McCoy

"One small thing was changed this time. A pet... died."
"A pet? Well, that wouldn't mean much in the course of time."
"It might, to some."
 - Spock and Kirk, after Spock returns to the present

"Every life comes to an end when time demands it. Loss of life is to be mourned but only if the life was wasted. I-Chaya's was not."
 - Spock

"Doctor McCoy, you do not know your good fortune. If the times were different, you would have to recalibrate for an Andorian."
"What's that supposed to mean? If that was supposed to be a joke, Spock, I have to remind you Vulcans don't tell jokes."
"Times change, doctor. Times change."
 - Spock and McCoy

Background information
Story
 The writer of "Yesteryear", Dorothy "D.C." Fontana, was a writer and story consultant for , and also served as the story editor and associate producer of . She said, "When I came to the [animated] show, I wanted to do at least one script." (Star Trek - Where No One Has Gone Before, p. 56) This episode is the only one she wrote for the animated Star Trek.
 Prior to the writing of this animated episode – with its portrayal of a boyhood Spock – a young version of him had been temporarily considered for inclusion as a regular character in Star Treks animated series, along with other child equivalents of the series' main characters. (The Art of Star Trek, pp. 42 & 43)
 Dorothy Fontana originally wanted this episode to be a "touch-back" to TOS.  She explained, "Yesteryear' resulted from my looking back at the things we had done on the [original] series." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 97) Fontana also realized that she wanted to feature Spock in the story, since he had always been her favorite of the main characters and was the focus of her favorite episodes from the ones she had written for TOS, such as  (which this episode ties into) and . (Star Trek - Where No One Has Gone Before, p. 56)
 Dorothy Fontana was excited about the prospect of exploring Spock's formative years, especially the early history of his familial relationships. "We could probe into these characters," she related, "and see the beginning of some of the trouble with Spock and Sarek, Amanda's problems back then and part of what made Spock Spock." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 97) At the time she plotted the installment, one of the questions Fontana asked herself – she having explored the conflicted state of relations between the adult Spock and Sarek in "Journey to Babel" – was, "How had that relationship been before, why did it evolve?" (Star Trek - Where No One Has Gone Before, p. 56)
 This outing is a sequel to the TOS episode . In fact, Dorothy Fontana once explained that this installment stemmed from "remembering the time portal from 'City on the Edge of Forever' [namely, the Guardian of Forever]." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 97) She kept thinking about that time portal. (Star Trek - Where No One Has Gone Before, p. 56) Fontana recollected, "I thought we could use [the Guardian] for a legitimate trip, but then have something happen so that Spock has to return to Vulcan to his childhood." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 97) The reuse of the Guardian of Forever from "The City on the Edge of Forever" was not only a tie-in to one of the most popular episodes of TOS but also minimized the need for exposition concerning time travel, which was important since Fontana already intended so much backstory, unrelated to the Guardian, to be in this episode. (TAS DVD text commentary) Indeed, she once referred to the Guardian of Forever as "an ideal way to put Spock back into his childhood, which was the story I wanted to do." 
 Dorothy Fontana pitched the story to Gene Roddenberry, who approved of the pitch. () Inspired by one of his guidelines, Fontana tried to utilize animation in such a way as to show a more expansive portrayal of the planet Vulcan than had previously been possible. She said, "I was mindful of Gene Roddenberry's rule that writers must take advantage of the enormous range animation gave us, in terms of 'sets' and aliens [...] Animation [...] would allow us to show the planet Vulcan any way I saw fit. Although it had been established in 'Amok Time' that most of the planet was desert, I wanted to depict other aspects of Vulcan." (Babel #5; Enterprise Incidents, issue 11, p. 26) Fontana also mused, "We never really could [show Vulcan] on the [original] series except, in 'Amok Time', we had a ceremonial circle, very small piece. We could show the whole planet of Vulcan, and a lot more of it, in the animation. So, that was one of the reasons I wanted to do the story."  Fontana concluded, "I realized that here was a way to tell a really nice Vulcan story in a way that would look good [...] It was an absolutely ideal way to [show Spock's backstory] [...] because of the freedom that the animation allowed us. For instance, we could show a great deal of ShiKahr and the surface of Vulcan, the desert, the city and environment of the home, and the strange creatures that lived there, the Le-matya and [...] the sehlat." ()
 The storyline involving the sehlat was not included in the episode's pitch. Dorothy Fontana noted, "I came up with that angle in the outline." () The concept of Spock having had a pet sehlat in his youth had already been established, however, the creature having been spoken about (though without Spock's particular sehlat being named) in "Journey to Babel". Fontana observed, "You [could now] finally get to see a sehlat, which could only be referred to in 'Journey to Babel'." (Star Trek - Where No One Has Gone Before, p. 57)
 Another influence on the sehlat I-Chaya was a cat that Dorothy Fontana owned, which was named Bobby McGee (after  that  popularized). (TAS DVD text commentary) Shortly after working on the episode, Fontana remarked, "As to who and what I-Chaya would be as a character, I decided he would be closely patterned on my large cat, Bobby McGee. While Bobby is not old and fat, he has the same affection, snuggle-ability, the fastest claws in the West, and complete disregard for orders. 'One word from me, and he does exactly as he pleases.' The statement fits both Bobby and I-Chaya." (Babel #5; Enterprise Incidents, issue 11, p. 27)
 The euthanizing of I-Chaya embodied a theme that Dorothy Fontana was eager to teach youngsters about. "I actually wanted to do a story that dealt with death," she admitted. "It just seemed to me that so many times children are not aware of death and then, when a pet dies, the child is devastated by it. The parents find it's difficult to explain the situation. And I wanted to touch on a way to deal with the subject." () Fontana additionally remembered, "[I] felt strongly about dealing with the death of a pet. It was a very serious thing for kids. We were trying to put across a lesson to children, that when it comes time for an animal to die, if he must go, it should be with dignity." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 97)
 Fearing that children would be "upset" by the depiction of euthanasia, NBC executives wanted the ending of this episode changed, but Dorothy Fontana refused and Gene Roddenberry supported her decision. (The Trek 25th Anniversary Celebration; TAS DVD text commentary, et al.) NBC's concerns were voiced by the network's Standards and Practices department, after the episode's outline was submitted to the network. () Andy Mangels explained, "[The death of Spock's pet sehlat] [...] was something that NBC was absolutely freaked out about. Like, how do you show the death of a pet, a child's pet?! How do you do that and not traumatize your audience?! But remember that Filmation had complete creative control, and [the deciding factor] was, if Gene Roddenberry approved it, the network had to go with it."  Years after the incident, Fontana reflected, "Gene said, 'Trust Dorothy, she'll handle it." (Star Trek - Where No One Has Gone Before, p. 56) NBC thereafter followed Roddenberry's advice, allowing the example of euthanasia to be shown. (TAS DVD text commentary) Noted Fontana, "I really appreciated his confidence in me. So I handled it, I think, in a sensitive way." (Star Trek - Where No One Has Gone Before, p. 56) However, not a word was directly passed from the network to Fontana herself, who later commented, "There was never any discussion at any time that the story was too adult for a children's audience." Having had past experiences of losing pets of her own, though, she was nevertheless aware of the feelings involved in such a loss and knew she had to handle the issue delicately and sensitively. ()
 The creation of the Vulcan city ShiKahr was influenced by D.C. Fontana considering where the eminent Ambassador Sarek might reside, a question she had pondered years beforehand. "Reasonably, he would maintain his residence in a city, probably the capital," Fontana noted. "Therefore, I created ShiKahr as the foremost city of Vulcan. I had visualized this city before in an early draft of 'Journey to Babel' [...] I still saw the city from the way I had in ." (Babel #5; Enterprise Incidents, issue 11, p. 26) Additionally, Fontana explained, "I went back to the description from that script and said 'let's do this now.' I wanted to see a city with parkways and trees, with growing things, and with unique spires." 
 According to D.C. Fontana, she intended the name "Vulcan's Forge" literally, due to the dangers existing therein. One of these perils was sucker vines, which were written into the script but could only be shown fleetingly, due to NBC censorship. (Babel #5; Enterprise Incidents, issue 11, pp. 26-27)
 Due to the limited time constraints of the narratives in Star Treks animated series, D.C. Fontana found there wasn't enough time for an in-depth exploration of what eventually happened to Thelin. She felt that answering the question of whether he was wiped from existence by Spock's time traveling might have been better addressed if this episode had been longer than half an hour. "This is ethical time traveling because it deals with characters," she observed. "If something is changed in another section of time, it causes a ripple effect, like a pebble thrown in a pond [....] Ideally, it [the story] would have dealt more with the characters. This is truly a Vulcan and a Spock story, so we had to go with it from that perspective." ()
 D.C. Fontana thoroughly enjoyed working on this episode. She enthused, "It was great." (Star Trek - Where No One Has Gone Before, p. 57) Fontana also felt compelled to tell this episode's tale, saying in retrospect, "It was an opportunity to tell a story that was very dear to my heart." ()

Script
 The first draft of this episode's script was submitted on . The final draft of the script, which included a page from that initial draft, was submitted on 20 April 1973, though some pages were revised three days later.
 The character names of Grey, Erickson, Bates, Stark, Sofek, Sepek, and Aleek-Om are not mentioned on-screen and are derived from the episode's script. Furthermore, the script named Aleek-Om's species as Aurelian. () In addition, the teleplay described the character as "a native of the planet , and he is a bird-like creature of blue-green hue."
 The first draft script of this episode contained an ultimately omitted scene extension that dealt with the aftermath of McCoy admitting that Spock wasn't familiar to him. In the omitted section, Spock replied, "I am not amused, doctor," to which McCoy answered, "I'm not being funny, mister." Angered, Kirk announced, "All right, Bones, I've had enough of this game," before he called for the Enterprise to beam them up. This scene was changed in the final draft script, replaced with a scene description that instead merely stated, "Kirk eyes McCoy curiously, puzzled," prior to Kirk calling for the beam-up, the wording of which was similarly very slightly different between those two drafts as well as between the final draft and the on-screen version.
 Another revision was in Kirk's response to Scott voicing surprise at seeing a Vulcan in the beamed-up group. Originally, instead of Kirk ordering Scott to "explain" himself, Kirk declared, "That will be enough, Mr. Scott." He went on to "angrily" say, "I'm ordering you to stop playing games with the dignity of my First Officer. I don't know who started it," at which point Kirk was interrupted by Thelin entering with an assurance to the captain that his dignity was "quite intact". In the final draft of the script, though, this dialogue was changed to Kirk commenting, "I don't know what's going on, but the First Officer of this ship will be treated with respect," at which point Thelin interrupted with an assurance to the captain that nobody had ever treated him otherwise.
 The first draft script specified that the Andorian character of Thelin was to have blue skin. This detail was retained in the final draft.
 The first draft script didn't, on the other hand, include Kirk saying to Spock, in reply to him concluding they were not in a game, "No... but if it's reality... what happened?" The same draft of the teleplay also left out a captain's log entry immediately thereafter. However, both portions of dialogue were in the final draft.
 In the first draft teleplay, a baffled Erickson revealed, during the subsequent briefing scene, that he couldn't find anything that might have "changed any time lines," though this phrasing was replaced (in the final draft) with "affected the future".
 The first draft script did not have Kirk tell Spock, "But I know who you are, and no one else aboard does. While we were in Orion's past, the time revision that took place here didn't affect me." However, this statement was written into the episode by the time the final draft was submitted.
 Chekov, described as an ensign, was in the first draft script, rather than Bates. By the time the final draft was issued, Bates had been created, replacing Chekov's role in the story.
 In the first draft script, a particular close-up of Spock was instructed to be shown but wasn't described. The shot was intended to portray him reacting to news that Sarek and Amanda had separated following the death of their son. In the final draft, a scene description specified, "A reaction on this... tightly held back... but surprises and pain." Also, Spock didn't react to news of Amanda's death by stating, "My mother," in the first draft, though he did say this in the final draft.
 In the first draft script, Spock described "the twentieth day of Tasmeen" as "the day my cousin saved my life in the desert." In the final draft, he concluded this sentence with "when I was attacked by a wild animal." Also, in both the first draft and the final draft of the teleplay, Spock referred to the name Selek as "a common name in my father's family," though this description is not in the episode. Another piece of dialogue included in both drafts but not the actual installment had Spock saying that none of his family, after they were visited by the Vulcan who had called himself Selek, had ever seen the man again.
 In the first draft script, Kirk told Spock, "This time, you were in Orion's past with us when Vulcan history was replayed – so you died as a boy." In the final draft, the same statement was refined to become, "This time, you were in Orion's past with us when the historians had the time vortex replay Vulcan history. You couldn't be in two places at once – so you died as a boy."
 In the first draft teleplay, Kirk asked the Guardian if it was possible for Spock to "repair the time line that has been broken". In the final draft, Kirk concluded this question with the words, "so all is the same as before?"
 As evidenced by the first draft script, Kirk originally told Spock, "You have to remember – to exist." In the final draft, Kirk instead cited the need for Spock to remember as being "for you... and your mother... to live."
 In the first draft teleplay, Thelin mused that, in Spock's timeline, he and and his "family" would continue to live. In the final draft, however, Thelin mentioned Spock and his "mother" living, rather than Spock's family.
 In the first draft script, Spock used the Guardian of Forever by instructing it, "Activate for planet Vulcan." This became, in the final draft, Spock more casually telling the Guardian, "I wish to visit the planet Vulcan."
 The first draft script didn't contain Spock's personal log at the start of the second act, though the final draft did.
 Both drafts noted the potentially vicious nature of sehlats. Although the sehlat seen here is commonly referred to as "Eye-chi-ah" in the final version of the episode, the name was phonetically spelled as "EE-chi-ah" in the installment's script, which is how Fontana much preferred the name to be pronounced. (Babel #5; Enterprise Incidents, issue 11, pp. 27, 29)
 In both drafts of the script but not in the episode itself, Sarek warned Young Spock that his "schoolwork has been disgraceful" and, in reply to Sarek admonishing the boy for having been "seen fighting in the street", Young Spock answered back, "Personal combat is not dishonorable." To that, Sarek retorted, "Brawling like a common deckhand off a freighter is," advice that Spock finally accepted, responding, "Yes, father." In the episode, this reply of Spock's is in response, instead, to Sarek telling him he has been witnessed quarreling in the street.
 The first draft script did not include Sarek advising Spock, "The time draws near when you will have to decide whether you will follow Vulcan or human philosophy. Vulcan offers much... no war... no crime... order, logic and control in place of raw emotion and instinct. Once on the path you choose, you cannot turn back." This dialogue was included in the final draft. It is in the actual episode, though, that iteration of the statement changes "emotion" to "emotions".
 A comment by Amanda Grayson wasn't in the first draft teleplay either. This was when she expresses that she hopes Spock will find his direction in life and goes on to observe, "I respect Vulcan and all its traditions. But it is a demanding life..."
 Sarek was originally to have told Spock he would not disappoint his father by failing the kahs-wan maturity test, adding, "Not if you are a Vulcan." In the final draft, this became, "Not if your heart and spirit are Vulcan."
 The script also declared that no moon was to be pictured in the Vulcan sky. (Babel #5; Enterprise Incidents, number 11, p. 27)

Cast and audio
 The dialogue from this episode that was spoken by Star Trek: The Animated Series regular cast was recorded when all of those cast members were together (the first time they had reunited since filming of TOS ended in January 1969). This recording session was at Filmation's studios in Reseda, California, in June 1973 (on or prior to the fourth of that month), and also included recording of the vocals for  and . (; The Star Trek Compendium, 4th ed., p. 143) Some of the cast's lines from this installment were rerecorded at a later date. (Babel #5; Enterprise Incidents, number 11, p. 27)
 With the addition of this episode, Mark Lenard (Sarek) became one of only three actors, besides the regulars, to play the same character on both this series and TOS. The other two were Roger C. Carmel (Harcourt Mudd) and Stanley Adams (Cyrano Jones).
 Before Mark Lenard was available for the episode, James Doohan was originally set to voice Sarek and actually recorded the character's lines of dialogue, over which Lenard's voice was thereafter looped. ()
 D.C. Fontana was pleased that Mark Lenard was available to voice the role of Sarek. She later remarked, "I was fortunate that [he was]," and went on to say that Lenard reprising the role "added weight to the story." ()
 Although the role of Amanda Grayson was most frequently played by Jane Wyatt (including an appearance in TOS), the character was voiced by Majel Barrett-Roddenberry here. D.C. Fontana explained, "Jane Wyatt was not available to do the voice of Amanda, so Majel Barrett did that characterization, pitching her voice to that of Ms. Wyatt's as closely as possible." () Similarly, even though the Guardian of Forever was originally voiced by Bartell La Rue (in "The City on the Edge of Forever"), James Doohan provided its voice for this episode. Quite unlike LaRue's rich, booming register, Doohan enunciated the Guardian's words in an elderly, quavering tone.
 This is one of two episodes of the animated Star Trek series that James Doohan provided the most voices for; in both this installment and , seven different characters are voiced by Doohan.
 According to D.C. Fontana, the practicalities of casting would have meant that trying to produce this as a live-action episode would have been extremely challenging. "It would have been far more difficult to do it live, because we would have had to have a young Spock, a young actor, and that was a difficult role," Fontana observed. "As it was, we managed to get a boy whose voice was quite good, and we were able to pull it off that way." 
 In 1973, a nine-year-old Billy Simpson was selected for the role of Young Spock. He later remembered first learning of the part; "My agent called with an audition at Filmation Studios for a new animated Star Trek show. Having not really been too aware of the original series, I didn't really appreciate how important this role could be!"  Simpson did have some awareness of the original Star Trek series, however, and – upon walking into Filmation Studios with his mother – he was surprisingly handed the script. () "I was given a rather extensive stack of sides to study in the waiting room (usually it's just one sample scene from the script) and then was called into a recording studio with director Hal Sutherland in the booth," explained Simpson. "He rolled tape as I recorded all of the isolated lines marked 'Young Spock' and that was it."  Simpson further remembered, "I was standing alone in the studio with my script pages on a stand. Sutherland was in the booth behind the glass and would prompt me to read each of my isolated lines [....] It seemed odd that I was reading so much for what was supposed to be merely an audition." ()
 Billy Simpson found the recording session challenging and, even though he was somewhat familiar with TOS, he struggled with emulating the portrayal of the adult Spock, which Hal Sutherland instructed him to do. "The only coaching he gave me," offered Simpson, "was to read the lines in a rather stilted way, as Mr. Spock would [...] I recall being rather uncomfortable at that direction, having difficulty distinguishing stilted 'Spock'-like interpretations from just plain bad acting! Since I wasn't hearing too many of my cue lines, it was difficult to respond naturally [...] I stumbled on the pronunciation of some of the more unusual names." ()
 Billy Simpson experienced difficulty with one particular line. "The preceding line stated something to the effect that it would be impossible to bring the injured I-Chaya to a healer," reflected Simpson. "My line should have had the inflection, 'I will bring a healer HERE,' but instead was 'I will bring a HEALER here.' Having been unaware of the context, that seemed the 'logical' reading." 
 Hal Sutherland liked the audio tape from Billy Simpson's audition to such a degree that he used it for the actual episode. (TAS DVD text commentary) "A day or two later, the agent called with the exciting news that I got the part!" Simpson exclaimed. "When asked when the recording date would be, I was told that I already did it!"  No reason for the audition tape's use in the episode was given to the baffled Billy Simpson. () He ultimately felt the "cold reading" quality of the tape was readily apparent from listening to the episode, including audible hesitation on "some of the unusual character names I had to pronounce," and he especially regretted his pronunciation of the line, "I will bring a healer here." 
 The change regarding the pronunciation of I-Chaya from "EE-chi-ah" to "Eye-chi-ah" happened amid the recording of the episode's vocals. Although the series' regular cast consistently said the former (and, according to D.C. Fontana, "proper") version during their recording of the script, Billy Simpson was told that the name was to be pronounced "Eye-chi-ah" and therefore repeatedly did so on his audition tape. D.C. Fontana concluded, "Rather than call him back to re-record at extra expense, the regular cast changed the name in pick-up lines read at the next recording session." (Babel #5; Enterprise Incidents, issue 11, p. 27)
 Hal Sutherland's son, Keith Sutherland, provided the voice of one of Young Spock's tormentors – namely, Sepek. (TAS DVD text commentary)
 The voice of the le-matya was a reuse of Godzilla's roar from Japanese movies. (Trek Navigator: The Ultimate Guide to the Entire Trek Saga, p. 272; )

Production

 As this episode was less action-oriented than a lot of the other animated Star Trek installments, Hal Sutherland experienced some difficulty with the episode's pacing. "I couldn't do anything with the camera or the action on that one," he later disclosed. "The pacing of it was quite different, and I was very surprised. I couldn't reach a stride of some excitement. It was a very emotional story, not an action story. And that was just my impression at the time. You would get all caught up in the momentum of what was happening on the show." 
 For the time vortex planet, Filmation's background artists recreated the planet's look from "The City on the Edge of Forever", painting the planet's backgrounds under the auspices of art director Don Christensen as well as director of color (and background director) Ervin Kaplan. (TAS DVD text commentary)
 Just as D.C. Fontana had planned, Filmation's artists were able, with expansive background paintings, to create grander vistas for the planet Vulcan than had been doable in TOS. (TAS DVD text commentary) Inspiring and witnessing the creation of the city ShiKahr was thrilling for Fontana. "Remember, up to that point, we'd never seen a Vulcan city of any kind," she pointed out. "I could describe the way it should be laid out, to my mind, and they drew it." (Star Trek - Where No One Has Gone Before, p. 57)
 The fact that a long shot of ShiKahr portrays a huge planetoid in the Vulcan sky, despite the script having specified that the planet Vulcan was to have no moon, was a mistake made during production. D.C. Fontana reflected, "Frankly, it was an error on the part of the animation house. The problem lies in the fact that, once past the story board, no one – no artist – ever [referred] to the script or descriptions [....] A preliminary drawing of the long shot included the huge orb in the sky. Both Gene Roddenberry and I noted, 'NO MOON!' on the sketch when it came to us for approval. Someone didn't get the word, and the final print shows that satellite in the Vulcan sky." (Babel #5; Enterprise Incidents, issue 11, p. 27) 
 Similarly, in contradiction to the episode's script, the Andorian Thelin was given a much paler complexion than those of known Andorians, due to the coloring practices of Filmation (director Hal Sutherland was color-blind); in the animated series, Andorians are sometimes grayish rather than blue, and Orions are commonly pale blue or yellowish instead of green.
 Evidently, Aleek-Om was another lifeform that was colorized differently than scripted, changing from the "blue-green hue" that is referred to in the teleplay.

 The episode's depiction of Spock's pet sehlat was influenced by the fact that, in "Journey to Babel", it is described as being akin to "a fat teddy bear" with six-inch fangs. According to the unauthorized reference book Boldly Writing (p. 5), the design of the sehlat I-Chaya was also inspired by a speculative article that was published in the  fanzine "Spockanalia 5". Boldly Writing states, "In this piece, the author looks at precedents in nature to see what sort of animal would have six-inch fangs. The author concludes, 'And so our portrait of the sehlat: a carnivore or just possibly tushed omnivore, general shape that of a giant panda, size on the order of an Alaskan brown bear, highly intelligent, and despite the six-inch fangs, of a patient and gentle disposition.... Question: did the sehlat belong to Spock, or was Spock in the care of the sehlat?' The Star Trek production staff read this article, and animators incorporated many of the suggestions into the drawing of the sehlat I-Chaya." 
 Another basis for the animated appearance of the sehlat was an illustration that fantasy and science fiction artist Alicia Austin drew for D.C. Fontana, who remembered, "Alicia Austin came up with several variations on what a sehlat looks like. I chose one of the early ones as a model – with modifications [....] I-Chaya has a different kind of tail, different shape of face, the broken fang." (Babel #5; Enterprise Incidents, issue 11, p. 27) Fontana purposefully instructed the Filmation artists to break off the creature's tooth, in order to give the animal a more aged appearance. 

Continuity
 D.C. Fontana once remarked that, due to her interest in making this installment a "touch-back" to TOS, the episode "does have a lot of references back."  Similarly, Billy Simpson said, "In retrospect it fills in several rather significant plot elements from the original series." () One element that Fontana thought importantly linked this episode with Star Trek of the time (such as both the original and animated series) was its depiction of the relationship between Kirk and Spock – for instance, Kirk being the only inhabitant of the alternate timeline who remembers the half-Vulcan officer. "There is a continuity of friendship between Kirk and Spock," opined Fontana, in , "binding this adventure into the total Star Trek concept [....] Without such ties to the original Star Trek concept, 'Yesteryear' would merely have been an interesting adventure that had no real relationship to the overall show – a dramatic diversion, but not consistent with series continuity." (Babel #5; Enterprise Incidents, issue 11, p. 29)
 Although Michael and Denise Okuda originally decided that the animated series would not be canonical, they also stipulated that this episode is the only exception, stating their reasoning as "partly because it is reinforced by material in '' [sic] and 'Journey to Babel', but also because of Fontana's pivotal role in developing the background for the Spock character in the original Star Trek series." () It is not only the Okudas who accept the events of this episode to be canonical; many other production staffers also do. (, et al.) Even Gene Roddenberry reportedly regarded the episode as canon. (Cinefantastique, Vol. 37, No. 2, p. 37)
 Spock's survival in the past required that he travel in time to save himself. Therefore, within the logic of the episode, the "normal" timeline is actually an altered timeline.
 In a production inconsistency, a low portion of the le-matyas head abruptly disappears during a close-up of the adult Spock's hands around its neck, during the nerve pinch that he uses to down the creature. A similar mistake occurs during the beam-in of Kirk and Spock in one of the Enterprises transporter rooms, near the end of the episode, as they also momentarily disappear, making their transport seem like a riskily stinted process.
 In another production inconsistency, when the landing party beams up from the Time Planet, they are wearing life-support belts, whereas they were not wearing them on the planet's surface.
 There are several production errors in the scene in the briefing room. At one point, Thelin's uniform color is blue, not black. Erickson has no identifying logo on his insignia. And Spock's insignia is transposed to his right side.
 Manny Coto considered this to be a very rare Star Trek outing. Twice referring indirectly to the installment, he opined, "The animated show [...] was one of the few shows to do a whole episode on Vulcan and do a really in-depth show about that." (Cinefantastique, Vol. 37, No. 2, p. 37)
 Andorians, as well as the characters of Sarek and Amanda, were reused, from "Journey to Babel", here (although Andorians also appear in ,  and , all three of which are TOS episodes that were produced and are set between this outing and "Journey to Babel"). D.C. Fontana clarified, "There are lots of references [to TOS] in 'Yesteryear', in terms of Spock's parents." () Andorians went on to appear in numerous productions after this one, as did Sarek and Amanda.
 The "family shrine" that Spock tells Sarek he is on a journey to was meant to be, according to D.C. Fontana, an allusion to the ceremonial grounds in "Amok Time". (Babel #5; Enterprise Incidents, issue 11, p. 27)
 D.C. Fontana's concerns over depicting the massive planetoid in Vulcan's sky were due to, in , Spock telling Uhura that "Vulcan has no moon." The theatrical edit of  later featured a similar large body, but for the director's cut of that film, it was decided to remove the orb.
 This is the first of three animated episodes that do not feature any scenes set on the bridge of the . The two subsequent episodes that exclude the location are  and .
 Similarly, this is the first of three episodes in which the character of Arex does not appear, the first of three that do not feature Uhura and the first of two wherein Sulu is absent. The two other installments that exclude Arex are "The Slaver Weapon" and "The Jihad", while Uhura appears in neither  nor "The Jihad", and Sulu is additionally absent from .
 Amanda's maiden name, Grayson, was first established here and later mentioned in .
 The city of ShiKahr essentially resurfaces on an okudagram in , as it includes the holoprogram title "Shi-Kar Desert Survival, Vulcan," which is also a subtler reference to Spock's kahs-wan. The city is again indirectly mentioned in , in reference to the Shi'Kahr Academy, and later serves as the namesake for the , seen but not mentioned in . A Vulcan city which looks very similar to ShiKahr is shown in new establishing shots used in the remastered version of "Amok Time".
 An okudagram that features in  references the Sepek Academic Scholarship, partly sharing its name with an aforementioned Vulcan child from this episode.
 The title of "healer," for a Vulcan physician, is referred to for Healer Senva in .
 Both Lunaport and the kahs-wan are mentioned in .
 Vulcan's Forge is later referenced in  and is the focus of a three-episode ENT arc: , , and . The location reappears in the first installment of that trilogy, but not in the other two episodes.
 The sehlat, having first appeared in animated form in this episode, was recreated in CGI for  and features in "The Forge".
 Spock's dialogue to his younger self, regarding logic offering "a serenity Humans seldom experience," is revisited by  in the  film , in which Spock likewise travels through time and meets . The film also depicts, with almost the exact same taunts, the confrontation of young Spock and three Vulcan bullies – though, in that version, Spock's physical response is much more effective.

Reception and aftermath
 On 4 June 1973, NBC made the announcement that Star Treks regular cast had reunited to record the script for this outing (as well as the teleplays for an additional two episodes). ()
 Even though this installment of the animated Star Trek series was the second to air in most of the United States, it was the first to be telecast in Los Angeles. As Sulu actor George Takei was running for a District Councilman post in that area, the broadcast of any episode in which he vocally features was considered unfair by his opponents, who demanded equal airtime. Therefore, this episode was swapped with series pilot ; unlike that installment, this one does not include Takei's vocals. (TAS DVD text commentary, booklet)
 D.C. Fontana, who did not see this episode until the day of its telecast, believed that fan reaction to the episode was initially negative. Shortly after its first airing, she remarked, "I'm sure when it was announced that my Star Trek animation script 'Yesteryear' was about Spock as a boy and included his sehlat, a lot of fans thought I had sold out to a kiddy show. Now that the episode has been on the air, I hope that notion has been disabused." (Babel #5; Enterprise Incidents, issue 11, pp. 26-27)
 Viewer response between the episode's announcement and  did not focus on the sehlats death; Fontana never had a conversation, during that time, with fans who spoke to her about the issue. () Also, despite NBC's initial nervousness over how controversial the euthanasia in this episode would prove, the television network never received a single letter of complaint about the story. (Star Trek - Where No One Has Gone Before, p. 57, et al.) Decades after the episode's broadcast, Fontana remarked, "We did not get one letter of complaint, so apparently we did it right."  Fan reaction did, however, concentrate on the thrill of viewing some of Spock's backstory. Fontana commented, "The response I get most often – and it has been consistent over the years – was that it was the one chance fans got to see Vulcan and had some insight into what made Spock 'Spock'." ()
 In 1974, D.C. Fontana addressed the issue of I-Chaya's demise in an article she wrote, which explained her intentions in writing the episode and defended the importance of the death in the context of the plot. She mused, "I-Chaya's death was absolutely necessary to the story. Part of Spock's training had to do with the facing of responsibilities and realities. One of the greatest weaknesses of children's programming on television, especially animation, is the presentation of total non-reality. Things do die – plants, pets and people. Is there anyone who, as a child, has not suffered the loss of a pet? In deciding that I-Chaya should die with peace and dignity rather than pain and suffering, Young Spock accepted reality and responsibility." (Babel #5; Enterprise Incidents, issue 11, p. 29) In a  interview, Fontana stated about the use of euthanasia here, "It was in the Vulcan way, because the argument for allowing the pet to go was its peace of mind; this was the kind thing to do, the peaceful and logical thing to do." () In a  video interview for StarTrek.com, Fontana thanked God that Gene Roddenberry had supported her decision to keep the euthanasia in the episode, saying the reason she was grateful was "because that, I think, was the power of that particular story, to say that it is kinder to let an animal in pain die with dignity than to keep them alive just for your sake. You know, it was a nice message, it was a moral tale." 
 In an interview from , D.C. Fontana related that this episode's story was still "very dear to [her] heart." () In her 2003 video interview, she additionally stated, "I looked at 'Yesteryear' not too long ago and I was amazed, myself, having not seen it for a while and not read my own script for a while, how much story we got into that and how complex the story was."  She also cited the episode as one of her favorites from the animated series, despite admitting that her opinion was prejudiced.  Fontana again expressed pride in the episode during a 2006 interview. ()
 Also in 2006, Fontana speculated about how this episode would have been as a live-action treatment. "Vulcan could probably have been visualized both with locations and on stages," she reckoned. "But I don't believe there was any way we could, with the technology of the time, have effectively done the sehlat and the Vulcan mountain lion. With the longer story time allowed by a one-hour live action show, I could have done more between [Spock and his parents] [...] and how Young Spock had a difficult time fitting into Vulcan life because of his Human heritage. But the action line would have had to be done differently because of the difficulty of creating the animals believably." ()
 The satisfaction with the decision to show I-Chaya's demise was exemplified by Filmation co-founder Lou Scheimer when he remarked, "A pet's death had never been done on a children's program, and it was touching and provocative. Dorothy [Fontana] was instrumental in making it so creative."  Scheimer also judged the depiction of euthanasia to have been "quite effective" and was happy with the installment in general. He went on to say, "Not only did it deal with something that was hardly ever dealt with before on Saturday morning, it was very moving and touching, and much more than just another science fiction story. It was a very Human story. We used alien people and animals to tell a very Human story, which is what Star Trek is all about, and why the animated version succeeded." 
 Due to having found the episode's pacing a challenge, Hal Sutherland felt that he had done an inadequate job on the outing. "And I felt that some of the younger viewers would have difficulty understanding the story," he continued. "It may have seemed somewhat tedious, and less action-packed. But so much for what I know, because it's become one of the most popular of the animated episodes." 
 At least in the opinion of Andy Mangels, this was an historically important episode. "It is the first animated show to show the death of a pet [...] and that's major; for that to happen on a Saturday morning show is seismic!" exclaimed Mangels. "It changed everything for Saturday morning television." 
 Star Trek: The Animated Series received a nomination during the First Annual Emmy Awards for Daytime Programming for the 1973-1974 season in the category of "Outstanding Entertainment Childrens Series," based on the submission of this episode.  Andy Mangels noted, "It was the first time that Star Trek had gotten that kind of attention, and it was the very first daytime Emmy Awards."  The following year, the TAS episode "How Sharper Than a Serpent's Tooth" won the series its only Emmy. Billy Simpson was under the erroneous impression that it was this episode that won the award. About three decades after the actual Emmy win (at a point when Simpson admitted that he found the details surrounding this episode's production were "a bit sketchy"), he recalled what had happened when he had met Hal Sutherland "a couple of years" after the making of this outing; "I recall being approached by Sutherland who proclaimed, 'Billy, do you know you won us an Emmy?!' He was referring to the award won by the animated Star Trek. 'Yesteryear' was the episode submitted for academy consideration."  Simpson also observed that, despite regretting some of his vocal work on the episode, the accuracy of his pronunciations "didn't seem to bother the Emmy voters, as I was told that 'Yesteryear' was the episode submitted and it won!" ()
 This episode did obtain a Filmcon Award for D.C. Fontana. 
 David Gerrold cited this as his favorite TAS episode, further commenting, "Brilliant storytelling, and some great background on Spock." 
 J.M. Dillard's reference book Star Trek - Where No One Has Gone Before (pp. 55-56) describes this installment as both "certainly one of the finest episodes of the animated show," and "the show's finest and most popular episode."
 Judith and Garfield Reeves-Stevens have also been highly appreciative of this installment, having found it to be emotionally effective. In their book The Art of Star Trek (pp. 49-43), they refer to it as a "moving episode" and an especially good example of "a few exceptions to the generally lack-luster animated episodes." In the DVD documentary "Drawn to the Final Frontier - The Making of Star Trek: The Animated Series", Judith Reeves-Stevens opines that the episode shows a "pretty poignant glimpse" of Spock's childhood.
 Another pairing of Star Trek production staffers who have exhibited enthusiasm for this outing are Michael and Denise Okuda. In their text commentary for the episode, they describe it as a "very Human tale," positively comment on its views of both the time vortex planet and Vulcan (describing the former as having "beautifully stylized backgrounds" and the latter as incorporating "imaginative vistas"), and state that it was "fortunate" that NBC complied with Gene Roddenberry's advice to "trust Dorothy." The Okudas' text commentary additionally enthuses, "D.C. Fontana's script proved to be a powerful yet sensitive treatment of a very emotional subject [....] [It] took advantage of animation's strengths to tell a story that would not have been possible for the original live-action show. The result is what many fans consider to be the finest episode of the animated Star Trek series." The Okudas also note that the prevalence of subsequently made productions that reuse elements from this episode reflects the outing's importance for the character of Spock. In their text commentary for "Amok Time", the Okudas refer to this episode as a "memorable visit to Vulcan." Likewise, the Okudas' text commentary for "The Forge" states that "Yesteryear" provided the Star Trek audience's "first tantalizing glimpse of a stylized Vulcan cityscape."
 The book Star Trek 101 (p. 48), by Terry J. Erdmann and Paula M. Block, cites this installment as "The Best Episode" of Star Trek: The Animated Series. The same publication also raves, "Yesteryear' deals with an interesting science fiction premise and carries the emotional significance found in the best Star Trek live-action episodes." Additionally, the book considers this episode to be either the only or most extreme exception to what it describes as the "flawed" nature of numerous episodes from the animated series.
 The unofficial reference book The Trek 25th Anniversary Celebration (p. 47), by James Van Hise, cites this episode, due to Mark Lenard reprising the character of Sarek herein, as one of three installments that, collectively, the book regards as "the plus side" of the animated Star Trek series (other such outings being  and ). The same book (p. 48) also defines this episode as "one of the few exceptions" to most of the other scripts for the series being "bland and uninteresting." The book goes on to say, "Yesteryear' is the only animated episode which, in its aired form, can be placed alongside some of the better live-action episodes without coming up a distant second [...] Stories dealing with actions deriving from characterization [were] considered too complex for Saturday morning. This is where 'Yesteryear' broke the mold."
 Similarly, the unauthorized reference book Beyond the Final Frontier (p. 57) enthusiastically refers to the installment as "the undoubted highlight" of the animated series. That book, which was first published in 2003, goes on to say, "[The episode] transcends the limitations of the animation to present a story that's rich, complex and moving, as well as being a fanboy wet dream. Even now, 'Yesteryear' ranks as one of the best Star Trek episodes."
 In , writer Philip Chien positively commented on this installment in a feature about the animated series, published in . He remarked, "What was particularly special about 'Yesteryear' was that it actually felt and looked like a live Trek episode."
 In the unofficial reference book Boarding the Enterprise: Transporters, Tribbles and the Vulcan Death Grip in Gene Roddenberry's Star Trek (p. 53),  described this as one of the "most notable" installments of the animated Star Trek series (in common with  and ).
 The editors of Trek magazine collectively scored this episode 4 out of 5 stars (a rating that they termed "very good"). (The Best of Trek #1, p. 109)
  rated this episode 5 out of 5 Starfleet arrowhead insignia and named it the best episode of TAS, also featuring Sarek as the "best guest star" of the series. Furthermore, the magazine referred to the installment as "a fabulous coming-of-age story" and stated, "It helped that 'Yesteryear' was not a simple retread of an original episode but had a fresh story with solid character development. [Mark] Lenard's distinctive voice helped this episode into the number one slot by reinforcing its continuity with the original series."
 In the book Trek Navigator: The Ultimate Guide to the Entire Trek Saga (pp. 271 & 272), co-writer Mark A. Altman rates this episode 3 and a half out of 4 stars (defined as "great") while fellow co-writer Edward Gross ranks the episode 4 out of 4 stars (defined as "classic!"). Altman describes the installment as "a superbly moving story," "a compelling Trek adventure," and "one of its most adult offerings" that nevertheless involves "some wonderful wit." He considers both Thalen's introduction and the moralistic plot point about I-Chaya's death to be successful. Altman also passes judgment on the outing as being "the best-acted episode of the animated series," and appreciates Mark Lenard's vocal return to the Sarek role. He criticizes, however, the casting of Majel Barrett-Roddenberry and James Doohan for the voices of Amanda and the Guardian of Forever. Gross regards the outing as "the perfect animated episode" and "the best of the series," additionally saying that the installment has "more resonance than most episodes." He also implies a belief that the episode's animation is the only facet that is less than perfect and bemoans the fact that the outing was not produced in live-action.
 In , UK magazine SciFiNow (issue 44, p. 026) cited this as the 24th best Star Trek episode. The magazine stated, "Featuring delicate levels of characterisation and exposition that wasn't intrusive, 'Yesteryear' was a fantastic piece of sci-fi writing."
 In 1974, D.C. Fontana re-used the plot of this installment for an episode of  entitled "Elsewhen". This was on the recommendation of David Gerrold who, at the time, was the story editor for Land of the Lost and who later commented, "I actually think the 'Elsewhen' episode has some strength to it that the 'Yesteryear' episode didn't, because it was trapped in the Star Trek universe." 
 According to Michael and Denise Okuda, this episode's portrayal of the planet Vulcan is of such a scale that it could not be equaled by later productions, "even the higher-budgeted Star Trek movies and spin-off TV shows [...] with matte paintings and location filming." (TAS DVD text commentary)
 By 1974, several people had asked what the orb in Vulcan's sky, in this installment, was and members of the production staff had had to dismiss it as being a sister planet. (Babel #5; Enterprise Incidents, number 11, p. 27)
 Following the appearance of the birdlike Aleek-Om in this episode, the same animated design was reused for the character of Tchar in Star Trek: The Animated Series first season finale, "The Jihad". The fact that, in that episode, the character is referred to as a Skorr rather than an Aurelian caused decades of speculative discussions about whether the two species were related.

Adaptations
 This episode was novelized by Alan Dean Foster in Ballantine Books' Star Trek Log 1 (along with  and ). Among numerous changes and additions made to this episode, as adapted in that book, are the facts that the name of Spock's pet sehlat is commonly spelled "Ee-chiya," that the natural habitat of the sehlat is "the cool, high forests of the north" and thus makes the animal's thick fur a heavy burden in the open desert, Spock's pronouncement that he can bring a healer to the wounded animal is notated as, "I can bring a HEALER here!" and no Vulcan-neighboring planets or planetoids are mentioned, with the novelization specifically stating, "Vulcan had no moon."
 This was the second of five Star Trek projects to be adapted into View-Master reels, and was retitled Mr. Spock's Time Trek. There was also a version for use with "talking" View-Masters. Both versions involved 21 pictures.
 Two animation cels, released by Filmation as part of a collection of cels from the animated Star Trek series, had this episode as its subject. One involved Aleek-Om, Kirk, Spock and McCoy standing near the Guardian of Forever, and showed Spock pointing at Aleek-Om with a hand including six digits. The other cel was of Young Spock riding I-Chaya, ready to battle the le-matya. In the collection, these cels were numbered 5 and 12, respectively. (A Trekker's Guide to Collectibles, p. 46; et al.)

Apocrypha
 The Star Trek: Myriad Universes story "The Chimes at Midnight" goes into greater detail about the alternate timeline in which Spock died, focusing mainly on the differences of the events in the films ranging from  to . The same story also states that Thelin is, in fact, half-Aenar, accounting for his particularly pale complexion.
 Other aspects of this episode have been included in a variety of Star Trek novels. For instance, the phrase "Vulcan's Forge" is not only the name of a desert area on Vulcan but also the title of one particular novel. The planet seen in Vulcan's sky here is named Charis or T'Khut in the novel Spock's World.

Video and DVD releases
 Released on LaserDisc in analog CX encoded audio, catalog number LV60754-6, 
 Re-released by Paramount and Pioneer: 
 UK VHS release (CIC Video): Volume 1, catalog number VHR 2535, 
 Released in Japan on LaserDisc, catalog number PILA-1406, 
 As part of the The Animated Series DVD collection:
 Region 1: 
 Region 2: 
 Re-release: 
 As part of the The Animated Series Blu-ray collection

Links and references
Starring
William Shatner as Capt. Kirk
Leonard Nimoy as Mr. Spock
And
DeForest Kelley as Dr. McCoy

Also starring
 Majel Barrett as
 Grey
 Amanda Grayson
 James Doohan as
 Guardian Voice
 Montgomery Scott 
 Thelin
 Erickson
 Bates
 Aleek-Om
 Vulcan healer

Guest stars
 Mark Lenard as Sarek
 Billy Simpson as young Spock
 Keith Sutherland as Sepek
 An unknown voice actor as Stark (uncredited)

 Background characters 
 Sofek

References
2230; 2235; 2237; 2264; 8877; adulthood; age; ambassador; Andorian; animal; annual physical; answer; antidote; apology; area; Aurelians; authority; barbarian; beauty; behavior; body; "Bones"; boots; captain; carry bag; case; charity; child; chit-chat; city; civilization; claw; conclusion; consciousness; courage; cousin; coward; crime; danger; ; day; death; ; desert; desert flyer; dignity; discipline; doctor; Earth; Earther; embarrassment; emotion; family; family history; family line; family shrine; Federation; Federation history; food; foothills; forefather; fountain; friend; god; graciousness; Guardian of Forever; Guardian ruins; healer; healing; heart; history; historian; home; honor; hospitality; Human; Human philosophy; humble; instinct; "in the course of time"; investigation; joke; kahs-wan; L-langon Foothills/L-langon Mountains; le-matya; liar; location; logic; Lunaport; marriage; maturity test; medical scanner; medicine; memory; Milky Way Galaxy; mind; mission; month; mourn; name; "no doubt"; "of course"; "on the part of"; orbit; order; ; Orions; pain; path; peace; perfection; pet; philosophy; physical; place; "point of view"; poison; practical joke; question; reality; regret; remarriage; research; respect; rite; road; Sarek's house; Sasak; schoolmate; screen; search; sehlat''; Selek; sensitivity; separation; serenity; shame; ShiKahr; shuttle accident; silence; spirit; Starfleet; Starfleet records; streets; street wear; sucker vine; survival test; sympathy; ; Tasmeen; tear; Terran; thing; timeline; time period; time plane; time planet; time revision; time vortex; tradition; trait; traveler; vacation; visit; visitor;  (planet); Vulcan (species); Vulcan desert soft-suit; Vulcan history; Vulcan neck pinch (neck pinch); Vulcan philosophy; Vulcan salute; Vulcan ship; Vulcan year; Vulcan's Forge; war; wardrobe section; warrior; warrior race; water; weapon; wish; witness; wound

Unreferenced materials 
; dust demon

External links 
 
 
 
 

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TAS episodes