No. 21 Squadron RAAF is a Royal Australian Air Force (RAAF) general reserve squadron. Formed in the mid-1930s as a unit of the part-time Citizen Air Force (CAF), it was mobilised for service during World War II, when it saw action against the Japanese as a fighter unit in the Malayan campaign, a dive bomber unit in the New Guinea campaign, and a heavy bomber unit in the Borneo campaign. After the war, the squadron continued to fly until 1960, when the CAF ceased flying operations. At that time, No. 21 Squadron converted to a non-flying support role, which it currently fulfils at RAAF Base Point Cook and RAAF Williams - Laverton.

The Royal Australian Air Force (RAAF) is the principal aerial warfare force of Australia, a part of the Australian Defence Force (ADF) along with the Royal Australian Navy and the Australian Army. Constitutionally the Governor-General of Australia is the de jure Commander-in-Chief of the Australian Defence Force. The Royal Australian Air Force is commanded by the Chief of Air Force (CAF), who is subordinate to the Chief of the Defence Force (CDF). The CAF is also directly responsible to the Minister for Defence, with the Department of Defence administering the ADF and the Air Force.