Auxiliaries are support personnel that assist the military or police but are organised differently from regular forces. Auxiliary may be military volunteers undertaking support functions or performing certain duties such as garrison troops, usually on a part-time basis. Unlike a military reserve force, an auxiliary force does not necessarily have the same degree of training or ranking structure as regular soldiers, and it may or may not be integrated into a fighting force. Some auxiliaries, however, are militias composed of former active duty military personnel and actually have better training and combat experience than their regular counterparts.

Historically, the designation auxiliary has also been given to foreign or allied troops in the service of a nation at war, most famously the eponymous Auxilia serving the Roman Empire. In the context of colonial troops, locally recruited irregulars were often described as auxiliaries.