Resistance to German occupation was widespread throughout Europe. In Western Europe the resistance movement centered around underground (secret) organizations that committed sabotage, aided downed Allied fliers, and worked with Allied intelligence agents. In Denmark, the Underground was so effective that it was able to evacuate or send into hiding almost all of the country's Jewish population just as the Germans were about to send Danish Jews to concentration camps.

In Italy, the Balkans, and parts of France, resistance took the form of guerrilla warfare. The guerrillas, often called partisans, sabotaged factories and military installations, attacked German and Italian troops, and encouraged the population to resist occupation. Communists were particularly active in the partisan movement; this helped the Soviet Union gain control of some countries after the war.

Partisan fighting was especially successful in Yugoslavia. Two rival guerrilla armies developed here, the Chetniks under Draja Mihailovich and the Partisans under Josip Broz, known as Tito. The effectiveness of these groups was hindered because they spent much of their time fighting each other instead of the Germans and Italians. Nonetheless, they were successful in tying down large numbers of Axis troops who might otherwise have been used on other fronts.