Desertion is the abandonment of a military duty or post without permission (a pass, liberty or leave) and is done with the intention of not returning. This contrasts with unauthorized absence (UA) or absence without leave (AWOL /ˈeɪwɒl/), which are temporary forms of absence.

In the United States Army, United States Air Force, British Armed Forces, Australian Defence Force, New Zealand Defence Force, Singapore Armed Forces and Canadian Armed Forces, military personnel will become AWOL if absent from their post without a valid pass, liberty or leave. The United States Marine Corps, United States Navy, and United States Coast Guard generally refer to this as unauthorized absence. Personnel are dropped from their unit rolls after thirty days and then listed as deserters; however, as a matter of U.S. military law, desertion is not measured by time away from the unit, but rather:

People who are away for more than thirty days but return voluntarily or indicate a credible intent to return may still be considered AWOL. Those who are away for fewer than thirty days but can credibly be shown to have no intent to return (for example, by joining the armed forces of another country) may nevertheless be tried for desertion. On rare occasions, they may be tried for treason if enough evidence is found.

There are similar concepts to desertion. Missing movement occurs when a member of the armed forces fails to arrive at the appointed time and place to deploy (or "move out") with their assigned unit, ship, or aircraft. In the United States Armed Forces, this is a violation of the Article 87 of the Uniform Code of Military Justice (UCMJ). The offense is similar to absence without leave but may draw more severe punishment.

Failure to repair consists of missing a formation or failing to appear at an assigned place and time when so ordered. It is a lesser offense within Article 86 of the UCMJ.

An additional duty status code — absent-unknown, or AUN — was established by the U.S. military in 2020 to prompt unit actions and police investigations during the first 48 hours that a service member is missing.

During the First World War, the Australian Government refused to allow members of the First Australian Imperial Force (AIF) to be executed for desertion, despite pressure from the British Government and military to do so. The AIF had the highest rate of soldiers going absent without leave of any of the national contingents in the British Expeditionary Force, and the proportion of soldiers who deserted was also higher than that of other forces on the Western Front in France.

In 2011, Vienna decided to honour Austrian Wehrmacht deserters. On 24 October 2014, a Memorial for the Victims of Nazi Military Justice was inaugurated on Vienna's Ballhausplatz by Austria's President Heinz Fischer. The monument was created by German artist Olaf Nicolai and is located opposite the President's office and the Austrian Chancellery. The inscription on top of the three step sculpture features a poem by Scottish poet Ian Hamilton Finlay (1924–2006) with just two words: all alone.