The Pat O'Leary Line was a resistance organization in France during the Second World War. The Pat O'Leary escape line helped Allied soldiers and airmen stranded or shot down over occupied Europe evade capture by Nazi Germany and return to Great Britain. Downed airmen in northern France and other countries were fed, clothed, given false identity papers, hidden in attics, cellars, and people's homes, and escorted to Marseille, where the line was based. From there, a network of people escorted them to neutral Spain. From Spain, British diplomats sent the escapees home from British-controlled Gibraltar. Many different escape lines were created in Europe of which the Pat Line was the oldest and one of the most important. Collectively, the many escape lines helped 7,000 Allied military personnel, mostly airmen, escape occupied France, Belgium, and the Netherlands. The Pat Line received financial assistance from MI9, a British intelligence agency.

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.