According to the United States Marines, a US Marine Corps officer is a college graduate who has earned and accepted, from the President of the United States, a commission as an officer in the Corps. Commissioned officers in the Marine Corps, or any branch of military service, are its leaders. There are three grades of commissioned officers: company grade officers, field grade officers, and general grade officers.
Company grade officers are second lieutenants, first lieutenants, and captains. A lieutenant generally commands a platoon of up to 40 – 45 Marines, and captains command companies consisting of up to four platoons. Company grade officers rarely develop strategy or operate autonomously; instead, they train their units in small-unit tactics and lead them as components of larger military units.
Field grade officers &emdash; majors, lieutenant colonels and colonels &emdash; command those larger units. Majors and lieutenant colonels usually command battalions, composed of five or six companies, while colonels are assigned to command regiments, usually made up of three battalions.
There are four types of general grade officer &emdash; brigadier general, major general, lieutenant general and general. General officers are assigned to lead larger units, such as brigades and divisions. In addition, they develop strategy and military operations doctrine, and consult with the civilian leadership of the nation's defense establishment in the formulation of national defense policy.
Not every Marine Corps officer commands a combat unit, of course. Many serve in staff and support positions in combat units. A rifle company, for instance, will have a first lieutenant or captain as its commanding officer, but another officer, typically a first lieutenant, will serve as executive officer (XO). A battalion has a larger cadre of officers, not the commanders of subordinate units, who are specifically assigned to duties like personnel management, logistics and intelligence. In addition, the Marine Corps has a wide variety of combat support organizations providing logistical and other support, which require the leadership of commissioned officers. There are also assignments other than leadership of ground combat or combat support units. A Marine Corps officer may serve as a pilot or a Naval Flight officer, and a lawyer must undergo Marine Corps officer training prior to training as a Marine Corps Judge Advocate (military attorney).
There are several ways to become a Marine Corps officer, but in all cases, commissions are only offered to college graduates. The Corps derives over a third of its officers from the Platoon Leaders Class, a program that offers summer training to college freshmen, sophomores, and juniors, which culminates in commissioning upon graduation. About one fourth of the Corps' new officers are trained in the Officer Candidate Course, a 10-week program for college seniors and recent graduates that also leads to a commission. The US Navy Reserve Officers Training Corps (ROTC) and the US Naval Academy together contribute almost a third of all new Marine Corps officers every year.
About ten percent of new Marine Corps officers are drawn from the enlisted ranks of the Corps itself. Enlisted Marines who already have earned a college degree attend Officer Candidates School. In other cases, the Marine Corps will permit an enlisted Marine to attend college and earn a degree. The Marine will undergo officer training during that time and will be offered a commission upon graduation.
Marine Corps officer training is a grueling regimen of physical and leadership training conducted at the Corps training facility at Quantico, Virginia. It includes the basic combat infantry training that new enlisted Marines undergo, as well as specialized leadership programs designed to enhance the trainees' ability to build teams, and solve problems.