Teaser
A teaser (also known as a ) is essentially the portion of an episode shown before the opening credits. As described by Michael and Denise Okuda, it is "a brief scene at the beginning of an episode, designed to make the viewer want to stay tuned." (ENT Season 3 DVD  text commentary)

The following memorandum was written on  by Gene Roddenberry, as a supplement to the Writer-Director Information Guide for the original series, describing the format of a typical episode:
a. Teaser, preferably three pages or less. Captain Kirk's Voice Over opens the show, briefly setting where we are and what's going on. This is usually followed by a short playing scene which ends with the Teaser "hook."

The "hook" of the teaser was some unexplained plot element that was alluded to in the teaser, which was intended to keep audiences interested enough in the show to dissuade them from changing stations while the titles roll. Star Trek writer David Gerrold, to tweak William Shatner on set, once told Shatner that he was writing a Star Trek episode in which Kirk lost his voice in the teaser (the hook), and didn't get it back until the .

Appearances
Teasers feature in almost all episodes of live-action Star Trek series, as well as those from animated series . The exceptions to this are:
 
 
 
 
 
 
 
 
 
 

 and  do not feature any teasers; nor does , with exception of its premiere episode .

Length
For live-action series prior to , as well as , the teaser is usually one to two minutes long, though can sometimes exceed five minutes. Three of the longest teasers from this era are ,  and , all clocking in at about seven minutes. The episode's title and production credits (guest stars, producers, writers and director) are typically held until the first segment after the opening titles (the lone exception is  in which the title and credits occur after a scene change within the teaser).

For , and other live-action series thereafter, the teaser regularly exceeds five minutes (excluding recap), with at least ten episodes exceeding eight minutes. The longest of these are  (12m 6s),  (12m 47s) and  (14m 19s). Due to their extended runtimes, these teasers can often be considered as "Act One" of the episode, although they still usually end with a traditional hook for the audience. These series do not feature title cards, with episode production credits either incorporated into the title sequence or held until the end credits.

Possibly the shortest teaser to appear in a Star Trek episode was from the  episode . This scene, in which a deranged, screaming T'Pol is carried into sickbay and restrained by Archer and Phlox, clocks in at just over 18 seconds. The second shortest teaser may be the one from s . This teaser, which has the Borg saying their trademark "greeting" just prior to their destruction by Species 8472, lasts just under 20 seconds.

Notes

Plot devices