In European militaries, a billet is a living-quarters to which a soldier is assigned to sleep. In American usage, it refers to a specific personnel position, assignment, or duty station to which a soldier can be assigned. Historically, a billet was a private dwelling that was required to accept a soldier.

Soldiers are generally billeted in barracks or garrisons when not on combat duty, although in some armies soldiers with families are permitted to maintain a home off-post. Used for a building, the term billet is more commonly used in British English; United States standard terms are quarters, barracks, Single (Soldier) Housing or Family Housing.

Originally, a billet (from French billet) was a note, commonly used in the 18th and early 19th centuries as a "billet of invitation". In this sense, the term was used to denote an order issued to a soldier entitling him to quarters with a certain person. From this meaning, the word billet came to be loosely used of the quarters thus obtained. The division of troops to organize their billeting was known as cantoning. Repeated petitions against the practice of billeting, starting in the 16th century, culminated in its outlawing in 1689 as an extension of a section of the Petition of Right 1628.

During wartime, civilians who have been evacuated from a city in danger of attack are billetted in communal shelters or in the homes of individuals. The practice of billeting evacuees was widespread in Britain during World War II, particularly during the Blitz when children and other non-essential persons in major cities were sent to rural areas for safety.

In European countries since the formation of regular forces, the Quartermaster was an occupation and a rank of the individuals responsible for the provision of sleeping quarters and other provisions for regular time troops. Train tickets are called 'billet' in many French influenced parts of the world including eastern Europe. Overnight transport by train was very common for soldiers and on the billet, the number indicating the location of the quarters would be specified. Later, during times of over-surging troops, more troops would be assigned to a train than its regular housing capacity. Thus, only those with a billet would be assigned to a room or quarter. Hence, the two became synonymous.^{[citation needed]}

One of the major grievances of the American colonists against the British government which led to the American Revolutionary War was the quartering of soldiers in civilian homes. As a result, the Third Amendment to the United States Constitution provides restrictions on the manner in which the federal government of the United States may require civilians to provide housing for American soldiers.

Billet can mean a specific personnel position, assignment, or duty station which may be filled by one person, most commonly used by the United States Navy, the United States Marine Corps, and the United States Coast Guard. Every person reporting aboard a ship or shore installation in the naval services is assigned a billet according to the unit watch, quarter and station bill, which shows the duties, stations and billet assignments for all crew members.

Billet can also refer to the position and weapons of the members of a unit. For example, the billets of a fireteam include a fireteam leader (M16), a rifleman (M16), an automatic rifleman (M249), and a grenadier (M16 with M203 grenade launcher).