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.

In countries with a volunteer military, reserve forces maintain military skills by training periodically (typically one weekend per month). They may do so as individuals or as members of standing reserve regiments—for example, the UK's Army Reserve. A militia, home guard, state guard or state military may constitute part of a military reserve force, such as the United States National Guard and the Norwegian, Swedish and Danish Home Guard. In some countries (including Colombia, Israel, Norway, Singapore, South Korea, Sweden, and Taiwan), reserve service is compulsory for a number of years after completing national service. In countries with conscription, such as Switzerland and Finland, reserve forces are citizens who have completed active duty military service but have not reached the upper age limit established by law. These citizens are subject to mandatory mobilization in wartime and short-term military training in peacetime.

In countries which combine conscription and a volunteer military, such as Russia, "military reserve force" has two meanings. In a broad sense, a military reserve force is a group of citizens who can be mobilized as part of the armed forces. In a narrow sense, a military reserve force is a group of citizens who have signed contracts to perform military service as reservists, who were appointed to positions in particular military units, and who are involved in all operational, mobilization, and combat activities of these units (active reserve). Other citizens who do not sign a contract (the inactive reserve) can be mobilized and deployed on an involuntary basis.

Some countries' 18th-century military systems included practices and institutions which functioned as a reserve force, even if they were not designated as such. For example, the half-pay system in the British Army provided the country with trained, experienced officers not on active duty during peacetime but available during wartime. The Militia Act 1757 gave Britain an institutional structure for a reserve force. Although contemporaries debated the effectiveness of the British militia, its mobilization in several conflicts increased Britain's strategic options by freeing regular forces for overseas theaters.

Reservists first played a significant role in Europe after the Prussian defeat in the Battle of Jena–Auerstedt. On 9 July 1807, in the Treaties of Tilsit, Napoleon forced Prussia to drastically reduce its military strength and cede large amounts of territory. The Prussian army was limited to a maximum of 42,000 men.

The Krumpersystem, introduced to the Prussian Army by military reformer Gerhard von Scharnhorst, gave recruits a brief period of training which could be expanded during wartime. Prussia could draw upon a large number of trained soldiers in subsequent wars, and the system was retained by the Imperial German Army into the First World War. By the time of the German Empire, reservists were given "war arrangements" after completion of their military service with instructions for the conduct of reservists in wartime.

In countries such as the United States, reservists are often former military members who reached the end of their enlistment or resigned their commission. Service in the reserve for a number of years after leaving active service is required in the enlistment contracts and commissioning orders of many nations.

Reservists can also be civilians who undertake basic and specialized training in parallel with regular forces while retaining their civilian roles. They can be deployed independently, or their personnel may make up shortages in regular units. Ireland's Army Reserve is an example of such a reserve.