Enlisted
In Starfleet, an enlisted crewman was one who had not completed the four-year Starfleet Academy course. Enlisted crewmen receive basic training, along with any specific courses required for their position at the Starfleet Technical Services Academy on Mars, before being posted. (; )

Other force structures, such as the Klingon Defense Force and the Bajoran Militia, have enlisted personnel in their ranks. 

As with commissioned officers, there was a rank structure among enlisted crew. () The term "non-commissioned officer" refers specifically to any enlisted personnel given authority over other personnel. () In Starfleet, this refers to any petty officer up to master chief. 

All commissioned officer ranks, from ensign upward, outrank all enlisted officers in the chain of command. () Additionally, chief petty officers, such as Miles O'Brien, wielded influence far beyond their place in the rank structure due to their extensive experience and skill and act as mentors to junior enlisted and commissioned officers. () Enlisted personnel may be promoted to receive a commission. ()

Rank structure

Naval ranks
 Master chief petty officer
 Senior chief petty officer
 Chief petty officer
 Petty officer 
 First class
 Second class
 Third class
 Crewman 
 First class
 Second class
 Third class

Ground-side and aviation ranks
 Master sergeant
 First sergeant
 Staff sergeant
 Sergeant
 Corporal
 Private/Airman

Ratings 
 Boatswain
 Chief
 Corpsman
 Engineer's mate
 Specialist
 Technician
 Yeoman

Background
Gene Roddenberry initially envisioned a Starfleet entirely composed of officers during the creation of . "Although the Enterprise is a military vessel," he said, "its organization is only semimilitary. The 'enlisted men' category does not exist. Star Trek goes on the assumption that every man and woman aboard the  is the equivalent of a qualified astronaut, therefore an officer." (The Making of Star Trek, p. 209) In a six-page memo he sent Gene L. Coon about the first draft script of  (sent on , while the episode had the working title "Court Martial on Star Base 811"), Roddenberry pointed out, "There is a feeling of an officer-enlisted man arrangement in this script. Actually, we've avoided terminology or playing of 'enlisted men' aboard our vessel. Every man aboard is a trained astronaut, even the cooks."

For the , costume designers created insignia for chiefs, petty officers, and even crewmen. In particular, Robert Fletcher's  uniforms had an entire series of enlisted rank insignia (p. 9). Some of these were reproduced (albeit incorrectly) in the . This is the origin of the "ables'man" term, which has not been used elsewhere in Star Trek. Fletcher's original costume notes actually referred to the rank of crewman as "able seaman", a term which has been used nowhere else in Star Trek.

Additionally, in Fletcher's personal notes (p. 11), he indicated that "there is no warrant rank," however, in a later  '"Star Trek" Cast Ranks' memo from Rick Berman to Bill Theiss, the intention during TNG was to have  warrant ranks, whereas it was stated that the "C.W.O." ("Chief Warrant Officer") was described as having "1 Black Disk" (pip). The  labels this enlisted rank insignia as "Chief Warrant Officer".

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Military ranks