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.

A reservist is a person who is a member of a military reserve force. They are otherwise civilians, and in peacetime have careers outside the military. Reservists usually go for training on an annual basis to refresh their skills. This person is usually a former active-duty member of the armed forces, and they remain a reservist either voluntarily, or by obligation. In some countries such as Israel, Norway, Finland, Singapore, and Switzerland, reservists are conscripted soldiers who are called up for training and service when necessary.