The Uniform Code of Military Justice (UCMJ, 64 Stat. 109, 10 U.S.C. Chapter 47), is the foundation of military law in the United States. The UCMJ applies to all members of the Uniformed services of the United States: the Air Force, Army, Coast Guard, Marine Corps, Navy, National Oceanic and Atmospheric Administration Commissioned Corps, and Public Health Service Commissioned Corps. The Coast Guard is administered under Title 14 of the United States Code when not operating as part of the U.S. Navy. However, commissioned members of the NOAA and PHS are only subject to the UCMJ when attached or detailed to a military unit or are militarized by presidential executive order.

Members of the military Reserve Components under Title 10 of the United States Code (Army Reserve, Navy Reserve, Marine Forces Reserve, and Air Force Reserve) or Title 14 of the United States Code, Coast Guard Reserve when not operating as part of the U.S. Navy, are subject to the UCMJ if they are either (a) active duty Full-Time Support personnel such as FTS or Active Guard and Reserve (AGR), or (b) traditional part-time reservists performing either (a) full-time active duty for a specific period (i.e., Annual Training, Active Duty for Training, Active Duty for Operational Support, Active Duty Special Work, One Year Recall, Three Year Recall, Canvasser Recruiter, Mobilization, etc.), or (b) performing Inactive Duty (i.e. Inactive Duty Training, Inactive Duty Travel and Training, Unit Training Assembly, Additional Training Periods, Additional Flying Training Periods, Reserve Management Periods, etc., all of which are colloquially known as "drills").

Soldiers and airmen in the National Guard of the United States are subject to the UCMJ only if activated in a Federal capacity under Title 10 by an executive order issued by the President. Otherwise, members of the National Guard of the United States are exempt from the UCMJ. However, under Title 32 orders, National Guard soldiers are still subject to their respective state codes of Military justice.

Cadets and midshipmen at the United States Military Academy, United States Naval Academy, United States Air Force Academy, United States Merchant Marine Academy, and United States Coast Guard Academy are also subject to the UCMJ. On the other hand, Reserve Officers' Training Corps (ROTC) cadets and midshipmen are by law exempt from the UCMJ (even while on active duty for training such as CTLT, LTC, LDAC, or while attending various training schools such as Airborne School, Air Assault School, Mountain Warfare School, etc.).

Members of military auxiliaries such as the Civil Air Patrol and the Coast Guard Auxiliary are not subject to the UCMJ. However, members of the Coast Guard Auxiliary can be called by the Commandant of the Coast Guard into the Temporary Reserve, in which case they become subject to the UCMJ.

Retired members of the uniformed services who are entitled to retirement pay are also subject to the UCMJ, as are retired reservists who are receiving hospital care in the VA system.