In some countries, such as the United States and the United Kingdom, members of the reserve forces are volunteers who have a part-time obligation. In other countries, such as the Republic of China and Israel, service in the reserves is compulsory for a number of years after one has completed their national service. Also, in some cases a militia could constitute part of a military reserve, such as the United States National Guard. A Home Guard is a specific type of reserve force that can only be activated in the event of invasion.

A military reserve force is different from a military reserve, which is a group of military personnel not committed to a battle so they are available to address unforeseen situations.