The work of the escape lines was labor-intensive. Typically, downed airmen were found, fed, clothed, given false identity papers, and hidden in attics, cellars, and people's homes by a network of volunteers. Airmen were then accompanied by guides, also volunteers, to neutral countries. The most common routes were from Belgium and northern France to Spain. Travel through occupied France was mostly by train, followed by a crossing on foot of the Pyrenees mountains into Spain with a local guide (usually paid). Once in Spain the airmen were assisted by British diplomats to travel to Gibraltar and then were flown back to the United Kingdom.^[5] An alternate route was to travel from the coast of Brittany to England via small boat.

Late in the war, especially after the Normandy Invasion on June 6, 1944, the escape lines turned more to sheltering airmen in place or in forest camps to await the arrival of the allied armies rather than helping the airmen to escape occupied Europe. Operation Marathon describes the forest camps.^[6]

About 2,000 soldiers, mostly British, and 2,000 British and 3,000 American airmen who had been shot down or crash landed in western Europe evaded German capture or escaped from German imprisonment during the war.^[7] Most of the soldiers were helped to evade capture because they were left behind in France after the Dunkirk evacuation of British forces in 1940. Most of the airmen were helped from 1942 to 1944 as the air war over Europe intensified. They were assisted by many different escape lines, some of them large and organized, others informal and ephemeral. The Royal Air Forces Escaping Society estimated that 14,000 volunteers worked with the many escape and evasion lines during the war. Many others helped on an occasional basis, and the total number of people who, on one or more occasions helped downed airmen during the war, may have reached 100,000. One-half of the volunteer helpers were women, often young women, even teenagers. Several of the most important escape lines were headed by women.^[8][9]

The work of escape line helpers was dangerous. Given the large numbers of helpers scattered over large areas, escape lines were relatively easy for the Germans to infiltrate. Thousands of helpers were arrested and more than five hundred died in concentration camps or were executed.^[10] The attrition of escape line leaders due to German arrest was much higher. In March 1943, only one 61-year old woman, Marie-Louise Dissard, remained free to re-invent the Pat O'Leary Line.^[11] In March 1944, only three of a dozen leaders of the Comet Line, the largest and most famous of the lines, were still alive and not in prison.^[12]

Initially, escape lines were self-financed by individuals in occupied countries. However, two UK clandestine organizations, mostly MI9 but also Section DF of the Special Operations Executive (SOE), financed the large escape lines and the U.S. clandestine organization MIS-X helped prisoners of war (POWs) escape from German POW camps.^[13]

The members of the escape and evasion lines were usually unarmed and did not participate in violent resistance to the German occupation. The motto of the Comet Line, was "Pugna Quin Percutias," which means "fight without arms." To maintain tight security, escape lines usually avoided contacts with armed resistance groups.^[14]

Organized escape and evasion lines operated in France, Belgium, the Netherlands, and Denmark. The number of airmen evading capture after being shot down or crash landing in western Europe was a small fraction of those killed or taken prisoner. For example, about 22,000 British and American airmen were killed or captured when being downed in France, but only 3,000 are recorded as having evaded capture by the Germans.^[15] Moreover, the percentage of airmen who evaded capture in France was higher than in other countries due to the proximity of the Spanish border to France and the short ocean passage to England. Nearly all the airmen downed in Germany were killed or captured, although a few escaped from prisoner of war camps and were helped to avoid re-capture by escape lines.^[16] Lesser-known escape lines operated in eastern Europe mainly to help Polish or Czech soldiers reach the Allies via the Baltic or Italy or to help Jews escape via the Balkans.^[17]

In addition to the escape lines listed below, many others were ephemeral, family-run affairs which have escaped the notice of history. One of the small lines which has received some attention is the Balfe Line near Amiens which was run by a husband, wife, and two teenage sons who hid escaping airmen in a shed in a World War I cemetery with the cooperation of cemetery employees. Many other small escape lines of a similar nature existed.^[18]

• Belgian National Movement
• Bordeaux-Loupiac Escape Network
• Bourgogne (Burgundy) Line
• Chauny Line
• Comet Line
• Denmark Escape Line
• Dutch-Paris Line
• Groupe Hoornaert-Dirix
• Hornoy-le-Bourg Line
• Francois Line
• Françoise Line
• Marie Claire Line
• Marie-Odile Line
• Oaktree Line
• Operation Marathon
• Operation Pegasus
• Operation Sherwood
• Pat O'Leary Line (Pat Line, O'Leary Line, PAO Line)
• Possum Line (possum = I can in Latin)
• Service EVA
• Smit-Van der Heijden Line
• Shelburne Line
• VAR line
• Vic Line