Supreme Headquarters Allied Expeditionary Force (abbreviated as SHAEF), was the command headquarters of the commander of Allied forces in North West Europe in 1944 and 1945. That post was held by General Dwight Eisenhower throughout its existence.

SHAEF was created in December 1943 when Eisenhower was recalled from the Mediterranean Theatre of Operations to the United Kingdom. Its staff took the outline plan for Operation Overlord created by Lieutenant General Sir Frederick Morgan , COSSAC (Chief of Staff to the Supreme Commander Allied Forces) and moulded it into the final version which was executed on 6 June 1944. That process was shaped by Eisenhower and the land forces commander for the initial part of the invasion, General Sir Bernard Montgomery.

SHAEF remained in the United Kingdom until sufficient forces were ashore to merit its transfer to France. At that point, Montgomery ceased to command all land forces, but continued as Comander in Chief of the British 21st Army Group (21 AG) on the easten wing of the Normandy bridgehead and the American 12th Army Group (12 AG) commanded by General Omar Bradley was on the western wing of the bridgehead. As the breakout from Normandy took place, the Allies launched invasion of southern France on August 15 1944 with the American 6th Army Group (6 AG) under the command of Lieutenant General Jacob L. Devers. During the invasion of southern France, the 6 AG was under the command of the Allied Forces Headquarters (AFHQ) of the Mediterranean Theatre of Operations, but after one month command passed to SHEAF. By this time the three Army Groups had taken up the positions on the Western Front in which they would remain until the end of the war. The British 21 AG to the North, the American 12 AG in the middle and the 6 AG to the South.

SHAEF commanded the largest number of American formations ever committed to one operation, along with substantial French, British and Canadian forces. It had three Army Groups under its command, which controlled a total of eight field armies (five American, one French, one British, one Canadian), and also First Airborne Army , a composite formation consisting of parachute troops and the aircraft used to drop them. SHAEF also controlled substantial naval forces during Operation Neptune, the assault phase of Overlord, and two tactical air forces (Ninth Air Force and Second Tactical Air Force). The strategic bomber forces in the UK came under its command during Operation Neptune.