The British Armed Forces are the armed forces of the United Kingdom of Great Britain and Northern Ireland.

Also known as Her Majesty's Armed Forces and sometimes legally as the Armed Forces of the Crown, the Armed Forces encompasses three professional uniformed services: the Royal Navy (including the Royal Marines), the British Army, and the Royal Air Force.

The Commander-in-Chief of Her Majesty's Armed Forces is the British monarch, at present Her Majesty Queen Elizabeth II, to whom members of the forces swear allegiance. Under British constitutional law, the armed forces are subordinate to the Crown, however this power is qualified by the requirement for parliamentary consent to the maintaining of a standing army and Parliament's approval of taxation and supply of funds for the armed forces. Under the 1689 Bill of Rights no standing army may be maintained during time of peace without the consent of Parliament and in modern times Parliament gives this consent every five years by passing an Armed Forces Act. Consistent with longstanding constitutional convention, the Prime Minister holds de facto authority over the use of the armed forces. The armed forces are managed by the Defence Council of the Ministry of Defence, headed by the Secretary of State for Defence.

The armed forces of a country are its government-sponsored defense, fighting forces, and organizations. They exist to further the foreign and domestic policies of their governing body, and to defend that body and the nation it represents from external and internal aggressors. In some countries paramilitary forces are included in a nation's armed forces. Armed force is the use of armed forces to achieve political objectives.

The study of the use of armed forces is called military science. Broadly speaking, this involves considering offense and defense at three "levels": strategy, operational art, and tactics. All three levels study the application of the use of force in order to achieve a desired objective.

In most countries the basis of the armed forces is the military, divided into basic Armed services. However, armed forces can include other paramilitary structures. For example, according to the Lithuanian law on organisation of system of national defence and military service (version actual from November 11, 2004), the Lithuanian armed forces in case of war also include border guards, public police service, parts of Lithuanian Riflemen's Union and guerrillas.