The Army Cadet Force (ACF) is a British cadet force that is sponsored by the British Army. It is not a pre-service organisation, but a significant minority of its members do go on to join the Army. Former cadets can also join the ACF as adult instructors, beginning as Sergeants or as Under Officers training to be Second Lieutenants. Usually officers run cadet detachments (based on an infantry platoon), but senior non-commissioned officers (e.g. Sergeant Majors) can run them as well. Every detachment is part of a company (for example, detachments have names such as 3 King's Cadet Platoon, affiliated to the King's Regiment). There are about 10-15 detachments per company and usually about 4-5 companies per cadet force, which may cover one or more counties or parts of counties. Each detachment is affiliated to a regular regiment or corps and wears its insignia.

The ACF is for cadets aged 13 to 18. It offers training in military skills, and helps cadets improve their self-esteem. Cadets join at the rank of Recruit, and are promoted up the same rank system as in the regular army. There also star levels for the amount of specialist training they have undertaken and successfully completed. Cadets wear brassards on the right arm showing their rank, star level and badges for other achievements such as marksmanship.

Cadets start off using the L98A1 Cadet General Purpose (GP) rifle, a variant of the SA80.They are later trained in the LSW (light support weapon), a longer barrelled version of the SA80. They may use the SA80 as well, either live or on SAT ranges.

On 1 April 2003, there were 7,640 officers and instructors and 43,550 cadets in the Army Cadet Force.