MI9, the British Directorate of Military Intelligence Section 9, was a secret department of the War Office between 1939 and 1945. During World War II it had two principal tasks: assisting in the escape of Allied prisoners of war (POWs) held by the Axis countries, especially Nazi Germany; and helping Allied military personnel, especially downed airmen, evade capture after they were shot down or trapped behind enemy lines in Axis-occupied countries. During World War II, about 35,000 Allied military personnel, many helped by MI9, escaped POW camps or evaded capture and made their way to Allied or neutral countries after being trapped behind enemy lines.

Escape and evasion lines in World War II helped people escape European countries occupied by Nazi Germany. The focus of most escape lines in Western Europe was assisting British and American airmen shot down over occupied Europe to evade capture and escape to neutral Spain or Sweden from where they could return to the United Kingdom. A distinction is sometimes made between "escapers" and "evaders". Most of those helped by escape lines were evaders.