Captain (United States O-3)

An Army captain generally serves as a battalion/squadron (cavalry) or brigade staff officer and may have an opportunity to command a company/battery (field and air defense artillery)/troop (cavalry). When given such a command, they bear the title company/battery/troop commander. U.S. Army Special Forces (12-member) Operational Detachments Alpha are also commanded by a captain, who has the title of "detachment commander."

Marine captains generally serve as staff officers in battalions/squadrons (aviation), regiments/aviation groups (MAG or MACG), or in MAGTFs (MEU and MEB) and may have an opportunity to command companies, batteries (artillery and air defense) or various types of detachments, with the title of commanding officer. In the Marine Raider Regiment, a captain, with the title of "team leader," commands a 14-man Marine Special Operations Team (MSOT). Marine captains also serve as executive officers (i.e., second-in-command) of infantry battalion weapons companies and some other larger combat logistics and aviation support units. Marine Aviation captains routinely serve as aircraft and air mission commanders, aircraft section and division leaders, aviation maintenance department division officers, and as officers-in-charge (OIC) of various combat logistics and aviation support functional and staff sections.

An Air Force captain's authority varies by group assignment. In an operations group, senior captains may be flight commanders while more junior captains may be heads of departments. In the maintenance or logistics and mission support groups they are almost always flight commanders. In the medical group, captains usually have limited administrative and command responsibility as captain is frequently the entry-level rank for most medical officers and dental officers.[1]

In the Space Force, a captain typically has authority over a flight, and is referred to as a flight commander.[2]

Captains of all four services routinely serve as instructors at service schools and combat training centers, aide-de-camps to general officers, liaison and exchange officers to other units, services, and foreign militaries, recruiting officers, students in advanced and graduate/post-graduate programs in Professional Military Education institutions and civilian universities, and on various types of special assignments.

In Army and Air Force medical units, captain is the entry-level rank for those possessing a medical degree, or a doctorate in a healthcare profession. Other health care professions including nurse anesthetists, pharmacists, optometrists, veterinarians, and physician assistants, among others may start as first lieutenants promotable upon completion of initial entry training.

In Army and Air Force Judge Advocate General Corps, lawyers with a Juris Doctor degree and membership in the bar of at least one U.S. state or territory are appointed captains, or first lieutenants promotable upon completion of initial entry training.

However, in Marine Corps Judge Advocates, after earning their commissions as unrestricted line officer second lieutenants, as well as earning an accredited Juris Doctor degree and passing a bar examination, enter active duty as first lieutenants and must complete the Marine Officer Basic Officer Course/The Basic School to qualify as rifle platoon commanders, before subsequently attending their MOS school prior to assignment to their first Marine Corps Judge Advocate billet and completing the minimum time in grade requirements for selection for and promotion to captain.