The Royal Canadian Air Force existed from 1924 to 1968, later designated "Canadian Forces Air Command" under the Canadian Forces, and then renamed to its original historic name of "Royal Canadian Air Force" in 2011. These are the squadrons that have served with Canada's air force since 1924.
Early squadrons.
These early squadron numbers have a history going back to the post-First World War Canadian Air Force which disbanded in 1920. These units were formed after the creation of the RCAF in 1924. 
100-series squadrons.
Established in Canada in the 1930s most of these units were either disbanded or reassigned a 400 series number for overseas service. 
400-series squadrons.
During the Second World War, the British Commonwealth Air Training Plan (BCATP) set out co-operation between the various British Commonwealth air forces. Under Article XV of the BCATP, RCAF squadrons were attached to and/or formed within British RAF operational formations. These squadrons were known as 'Article XV squadrons'. To avoid confusion with squadron numbers of other Commonwealth nations, RCAF units were assigned squadron numbers from 400 to 449. Not all squadrons up No. 449 were formed, however. This squadron series continued through the post-war years, and the majority of current Royal Canadian Air Force operational squadrons are still numbered in the 400s due to their legacy as wartime units. For this historical reason current squadrons are listed here with their original RCAF names. 
600-series squadrons.
During the Second World War pilots who served in the 600-series RCAF squadrons were recruited from the Royal Canadian Artillery in England and Italy, and trained to fly at No. 2 Elementary Flying Training School RAF Cambridge (England), completing their operational flying training at 43 O.T.U. (RAF Andover). Observers were trained at Larkhill (England); these were selected 'Other Ranks' from the Royal Canadian Air Force and Royal Canadian Artillery. The three Canadian 'air observation post' squadrons operated under the command of No. 70 Group RAF, RAF Fighter Command; the first two squadrons saw action while serving with No. 84 Group RAF, RAF Second Tactical Air Force.
Post-war squadrons.
Subsequent to the Second World War the RCAF expanded their squadron numbers to include 444–450. No. 450 was also a Royal Australian Air Force squadron during the war and the Canadian squadron duplicated the number by error, which was discovered when No. 450 Squadron RCAF was formed in 1968. 
References.
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