Often referred to as the backbone of the Corps, enlisted Marines with pay grades of E-4 and E-5 are non-commissioned officers (NCOs). Staff NCOs are career Marines serving in grades E-6 through E-9. Together they are responsible to the commanding officer for the welfare, morale, discipline, and efficiency of Marines in their charge. The ranks of E-8 and E-9 each have two ranks per pay grade, with distinct responsibilities. Gunnery sergeants (E-7) indicate their preferred promotional track on their annual evaluations. First sergeants (E-8) and sergeants major (E-9) serve as senior enlisted advisors assisting the commanding officer in matters of discipline, administration, and the morale and welfare of the unit. First sergeants serve as the senior enlisted Marine in a company, battery, or other unit at similar echelon, while sergeants major serve the same role in battalions, squadrons, or larger units. Master sergeants (E-8) and master gunnery sergeants (E-9) provide technical leadership as occupational specialists in their specific MOS. The sergeant major of the Marine Corps is the senior enlisted Marine of the entire Marine Corps, personally selected by the commandant. The sergeant major of the Marine Corps and the Marine gunner are the only billets which rate modified rank insignia in place of traditional rank insignia.
Warrant officers specialize in their respective fields, providing leadership and training to the Marines in their military occupational specialty. A warrant is approved by the secretary of the Navy for a sergeant (E-5) or staff non-commissioned officer (E-6 to E-9) to be appointed a warrant officer. Warrant officers become commissioned officers at the rank of Chief Warrant Officer 2, though they will generally serve as technical advisors, providing expertise to commands and organizations in their field. A chief warrant officer serving as an infantry weapons officer also carries the title, "Marine Gunner", which does not replace his rank. However, a Marine Gunner replaces the chief warrant officer insignia on the left collar with a bursting bomb insignia.
Commissioned officers are college graduates who have earned and accepted an appointment issued in the name of the president of the United States. Their commission gives them the responsibility of leading Marines as they defend the Constitution of the United States. Commissioned officer ranks are further subdivided into generals, field-grade, and company-grade officers. The commandant of the Marine Corps and the assistant commandant of the Marine Corps are four-star ranks.