The Chetniks, formally the Chetnik Detachments of the Yugoslav Army, and also the Yugoslav Army in the Homeland and the Ravna Gora Movement, was a Yugoslav royalist and Serbian nationalist movement and guerrilla force in Axis-occupied Yugoslavia. Although it was not a homogeneous movement, it was led by Draža Mihailović. While it was anti-Axis in its long-term goals and engaged in marginal resistance activities for limited periods, it also engaged in tactical or selective collaboration with Axis forces for almost all of the war. The Chetnik movement adopted a policy of collaboration with regard to the Axis, and engaged in cooperation to one degree or another by both establishing a modus vivendi and operating as "legalised" auxiliary forces under Axis control. Over a period of time, and in different parts of the country, the movement was progressively drawn into collaboration agreements: first with the puppet Government of National Salvation in the German-occupied territory of Serbia, then with the Italians in occupied Dalmatia and Montenegro, with some of the Ustaše forces in northern Bosnia, and, after the Italian capitulation in September 1943, with the Germans directly.

The Yugoslav Partisans, or the National Liberation Army, officially the National Liberation Army and Partisan Detachments of Yugoslavia, was the communist-led anti-fascist resistance to the Axis powers in occupied Yugoslavia during World War II. Led by Josip Broz Tito, the Partisans are considered to be Europe's most effective anti-Axis resistance movement during World War II.