In Starfleet, chiefs were subordinate to all commissioned officers, but they generally had far more experience and practical knowledge than ensigns and lieutenants, and were expected to act as mentors to them and to the junior enlisted assigned. (DS9: "Starship Down", "The Ship") As such, chiefs were often among the actual leaders in a Starfleet organization, and could even hold the authoritative position of Chief of Operations. (DS9: "Emissary") Chiefs served with distinction in most Starfleet branches and specialties, but were particularly visible in engineering and other technical trades. (DS9: "Hippocratic Oath")
During the late 23rd century Starfleet chief petty officers were also defined as senior chief petty officer and master chief petty officer. (Star Trek II: The Wrath of Khan–Star Trek VI: The Undiscovered Country)
When Sergey Rozhenko spied Miles O'Brien as the transporter chief aboard the USS Enterprise-D in 2367, Rozhenko was excited to meet another chief petty officer. When O'Brien introduced himself, he acknowledged Rozhenko as "sir", to which Rozhenko told him not to call him "sir," as he used to work for a living. Sergey's wife, Helena, then told O'Brien that he was just joking and that Sergey's proudest moment was when their son, Worf, had earned his commission. Sergey then added, "Imagine. An old enlisted man like me raising a boy to be an officer!" (TNG: "Family")
Several years later, O'Brien was identified by a Jem'Hadar as the rank of "chief petty officer," or "what Starfleet refers to as a "noncom,"" adding "you must have a great deal of experience […] that makes you a priority target." (DS9: "Hippocratic Oath")
List of chief petty officers
- Master CPO
- Senior CPO
|Enlisted personnel||Commissioned officers||Generals and flag officers|
|ENS||LT JG||LT||LT CMDR||CMDR||CAPT||FCAPT||CDORE||RADM||VADM||ADM||FADM|
|2nd LT||1st LT||CAPT||MAJ||LT COL||COL||BRIG||MAJ GEN||LT GEN||GEN||MARSHAL|
|Services:||Andorian • Bajoran • Cardassian • Ferengi • Human (CSA • Third Reich • UE MACO • USA) • Jem'Hadar • Klingon • Romulan • United Earth/Federation Starfleet • Vulcan|
Mirror universe: Terran Empire Starfleet • Terran Empire MACO
Personnel of this grade can be addressed simply as "chief".
As a traditional naval grade, CPO was senior to a petty officer, and was the highest enlisted grade in organizations like the US Navy. In comparison to infantry ranking systems, this rank was approximately equivalent to the more senior sergeant grades.
Scripts for "The Cage" described Garison's rank stripe as "first crewman" and "bridge CPO."
In Star Trek IV: The Voyage Home, a chief petty officer contacts the CDO Commander Rogerson, regarding a power drain in the energy systems of the aircraft carrier Enterprise. The CPO's voice is not heard on screen, but the commander responds to the question "Yes, chief. We're tracking that too."
Information pertaining to the Star Trek films "senior chief petty officer" and "master chief petty officer" ranks originated from Robert Fletcher's personal costuming notes (p. 9, seen here) and later confirmed in multiple lots sold in the It's A Wrap! sale and auction. Props designated as "master chief petty officer, second class" were apparently designed and created, but did not appear on Fletcher's notes, were also among the costume props sold in It's A Wrap!. 
Additionally, the revised shooting script for Star Trek IV was prefaced with several production notes, including one that stated: "Please note that Master Chief Petty Officer Rand now first appears in Scene 25 and in all subsequent Starfleet Command scenes," replacing the previous script note reference to her being a lieutenant. A similar note appeared during the production of Star Trek: The Motion Picture, which originally listed her as an ensign, but was revised to chief (no sleeve stripes), due to her position as the transporter chief.
The first appearance of Miles O'Brien's chevrons-and-dots insignia was "Hippocratic Oath", which Miles O'Brien (and in one case, Dorian Collins) would wear through the end of the series. Several O'Brien uniforms with this insignia were sold as part of the It's A Wrap! sale and auction.