The British Militia was the principal military reserve force of the Kingdom of Great Britain. Militia units were repeatedly raised in Great Britain during the Georgian era for internal security duties and to defend against external invasions. The Militia Act 1757, passed by the Parliament of Great Britain after the outbreak of the Seven Years' War, led to the rapid expansion of the British Militia in order to defend from potential French invasions. In the Kingdom of Ireland, a client state of Great Britain, the equivalent force was the Irish Militia, which saw heavy service in the Irish Rebellion of 1798 alongside British militia units. The existence of militia units in Great Britain and Ireland played an important role in freeing regular troops from the British and Irish establishments for overseas service.

A military reserve force is a military organization whose members (reservists) have military and civilian occupations. They are not normally kept under arms, and their main role is to be available when their military requires additional manpower. Reserve forces are generally considered part of a permanent standing body of armed forces, and allow a nation to reduce its peacetime military expenditures and maintain a force prepared for war. During peacetime, reservists typically serve part-time alongside a civilian job, although most reserve forces have a significant permanent full-time component as well. Reservists may be deployed for weeks or months-long missions during peacetime to support specific operations. During wartime, reservists may be kept in service for months or years at a time, although typically not for as long as active duty soldiers.