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"What's the matter, Chief?"
"It just occurred to me. As soon as that kid graduates from the Academy, I'm going to have to call him sir."
Julian Bashir and Miles O'Brien, 2371 ("Facets")

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. (TNG: "The Drumhead", "Eye of the Beholder"; DS9: "Shadowplay", "Starship Down")

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. (DS9: "Starship Down") The term "non-commissioned officer" refers specifically to any enlisted personnel given authority over other personnel. (DS9: "Hippocratic Oath") 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. (DS9: "Facets", "Hippocratic Oath") 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. (DS9: "Starship Down", "The Ship") Enlisted personnel may be promoted to receive a commission. (DS9: "Once More Unto the Breach")

Quick Answers

What training do enlisted personnel receive in Starfleet? toggle section
Starfleet enlisted personnel undergo basic training specific to their roles, unlike officers who receive comprehensive training. This training occurs at places such as the Starfleet Technical Services Academy on Mars. The entry-level enlisted rank is crewman, and these personnel operate under officer command on Starfleet ships or facilities.
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How does one become an enlisted crewman in Starfleet? toggle section
Becoming an enlisted crewman in Starfleet involves completing basic training and specific courses at the Starfleet Technical Services Academy on Mars. Enlisted personnel do not attend the four-year Starfleet Academy. After training, they are assigned to starships or outposts.
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What is the role of enlisted personnel in Starfleet? toggle section
Enlisted personnel in Starfleet operate under officer command, serving on ships or facilities. They receive role-specific training, unlike officers who undergo comprehensive training. The Starfleet Technical Services Academy on Mars trains enlisted personnel. The lowest enlisted rank is crewman. Enlisted personnel can lead departments, with authority akin to officers, commanding both officers and enlisted personnel.
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Where do enlisted crewmen receive their training in Starfleet? toggle section
Enlisted crewmen in Starfleet train at the Starfleet Technical Services Academy on Mars. They do not attend the four-year Starfleet Academy but receive basic training and specific courses for their roles before assignment.
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How do enlisted ranks differ from officers in Starfleet? toggle section
In Starfleet, enlisted personnel differ from officers in rank and command. Enlisted ranks, including petty officers, serve under commissioned officers, starting from ensign. Non-commissioned officers have authority over other enlisted members but are outranked by all officers. Enlisted personnel do not need a commission from Starfleet Academy, unlike officers who start as ensigns. Chief petty officers, such as Miles O'Brien, can have significant influence despite their rank.
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Rank structure

Naval ranks

Ground-side and aviation ranks

Ratings

Background

Gene Roddenberry initially envisioned a Starfleet entirely composed of officers during the creation of Star Trek: The Original Series. "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 USS Enterprise 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 TOS: "Court Martial" (sent on 15 August 1966, 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."

Robert Fletcher's original costume notes, page 9.

Robert Fletcher's original costume notes, page 9.

For the Star Trek films, costume designers created insignia for chiefs, petty officers, and even crewmen. In particular, Robert Fletcher's Star Trek II: The Wrath of Khan uniforms had an entire series of enlisted rank insignia (p. 9). Some of these were reproduced (albeit incorrectly) in the Star Trek Encyclopedia. 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 2 June 1987 '"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 Star Trek Encyclopedia, 4th ed., vol. 2, p. 369 labels this enlisted rank insignia as "Chief Warrant Officer".