The Chetniks, formally the Chetnik Detachments of the Yugoslav Army, and also the Yugoslav Army in the Homeland and informally colloquially 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 Chetniks were active in the uprising in the German-occupied territory of Serbia from July to December 1941. Following the initial success of the uprising, the German occupiers enacted Adolf Hitler's formula for suppressing anti-Nazi resistance in Eastern Europe, a ratio of 100 hostages executed for every German soldier killed and 50 hostages executed for every soldier wounded. In October 1941, German soldiers and Serbian collaborators perpetrated two massacres against civilians in Kraljevo and Kragujevac, with a combined death toll reaching over 4,500 civilians, most of whom were Serbs. This convinced Mihailović that killing German troops would only result in further unnecessary deaths of tens of thousands of Serbs. As a result, he decided to scale back Chetnik guerrilla attacks and wait for an Allied landing in the Balkans. While Chetnik collaboration reached "extensive and systematic" proportions, the Chetniks themselves referred to their policy of collaboration as "using the enemy". The political scientist Sabrina Ramet has observed, "[b]oth the Chetniks' political program and the extent of their collaboration have been amply, even voluminously, documented; it is more than a bit disappointing, thus, that people can still be found who believe that the Chetniks were doing anything besides attempting to realize a vision of an ethnically homogeneous Greater Serbian state, which they intended to advance, in the short run, by a policy of collaboration with the Axis forces".

The Chetniks were partners in the pattern of terror and counter-terror that developed in Yugoslavia during World War II. They used terror tactics against Croats in areas where Serbs and Croats were intermixed, against the Muslim population in Bosnia, Herzegovina and Sandžak, and against the Communist-led Yugoslav Partisans and their supporters in all areas. These tactics included the killing of civilians, burning of villages, assassinations and destruction of property, and exacerbating existing ethnic tensions between Croats and Serbs. The terror tactics against the non-Serb population in the NDH were, at least to an extent, a reaction to the massacres of Serbs carried out by the Ustaše, however the largest Chetnik massacres took place in eastern Bosnia where they preceded any significant Ustaše operations. Croats and Bosniaks living in areas intended to be part of Greater Serbia were to be cleansed of non-Serbs regardless, in accordance with Mihailović's directive of 20 December 1941. The terror against the communist Partisans and their supporters was ideologically driven. Several historians regard Chetnik actions during this period as constituting genocide. Estimates of the number of deaths caused by the Chetniks in Croatia and Bosnia and Herzegovina range from 50,000 to 68,000, while more than 5,000 victims are registered in the region of Sandžak alone. About 300 villages and small towns were destroyed, along with a large number of mosques and Catholic churches.

The etymology of the word Chetnik is obscure. Some believe it to have developed from the Turkish word çete, meaning "to plunder and burn down", while another view is it was inherited from the Proto-Slavic četa + nik, meaning "member of a company". The word may also derive from the Latin word coetus ("coming together" or "assembly"). The suffix -nik is a Slavic common personal suffix, meaning "person or thing associated with or involved in".

The first use of Chetnik to describe members of army and police units appeared around the mid-18th century.^{[citation needed]} Matija Ban used the word Chetnik in 1848 in terms of the need to establish armed units outside the Principality of Serbia to act in opposition to Ottoman rule following the rise of nationalism in the Balkans. At end of the 19th century, the term was extended to members of military or paramilitary organizations with Serb ethnonationalist aims. Dating from 1904, the Serbian word četnik was commonly used to describe a member of a Balkan guerrilla force called a cheta (četa/чета), meaning "band" or "troop". Today, the word Chetnik is used to refer to members of any group that follows "the hegemonic and expansionist politics driven by Greater Serbia ideology".

Small-scale rebellious activity akin to guerrilla warfare has a long history in the South Slav-inhabited lands, particularly in those areas that were under Ottoman rule for a long period. In the First Serbian Uprising which began in 1804, bandit companies (hajdučke čete) played an important part until large-scale fighting gave the Ottomans the upper hand and the uprising was suppressed by 1813. A second rebellion broke out two years later, and guerrilla warfare was again utilised to significant effect, assisting in the establishment of the partially-independent Principality of Serbia, which was expanded significantly in 1833 and became fully independent in 1878. Throughout this period and until the end of the 19th century interest in guerrilla warfare remained, with books on the subject being commissioned by the Serbian government and published in 1848 and 1868. Four years after independence, the principality became the Kingdom of Serbia.

Between 1904 and 1912, small groups of fighters who had been privately recruited, equipped and funded in Serbia, travelled to the region of Macedonia within the Ottoman Empire with the aim of releasing the area from Ottoman rule and annexing it to Serbia, regardless of the wishes of the local population. These groups were, in the main, commanded and led by officers and non-commissioned officers on active duty in the Royal Serbian Army, and the Serbian government soon took over the direction of these activities. Similar forces had been sent to Macedonia by Greece and Bulgaria, who also wished to integrate the region into their own states, resulting in the Serbian Chetniks clashing with their rivals from Bulgaria as well as the Ottoman authorities. Except for the social democratic press, these Chetnik actions were supported in Serbia and interpreted as being in the national interest. These Chetnik activities largely ceased following the 1908 Young Turk Revolution in the Ottoman Empire. The Chetniks were active in the Balkan Wars of 1912–1913; during the First Balkan War against the Ottomans they were used as vanguards to soften up the enemy ahead of advancing armies, for attacks on communications behind enemy lines, for spreading panic and confusion, as field gendarmerie and to establish basic administration in occupied areas. They were also put to good use against the Bulgarians in the Second Balkan War. After the Balkan Wars, Chetniks bands were used in the pacification of the new areas of Serbia gained during the wars, which occasionally involved terrorising civilians.

As they had proven valuable during the Balkan Wars, the Serbian army used Chetniks in World War I in the same way, and while useful they suffered heavy losses. At the end of the Serbian campaign of 1914–1915, they withdrew with the army in the Great Retreat to Corfu and later fought on the Macedonian front. Montenegrin Chetniks also fought against the Austro-Hungarian occupation of that country. In late 1916, new Chetnik companies were being organised to fight in Bulgarian-occupied southeastern Serbia. Concerned about reprisals against a large-scale uprising, the Serbian army sent the veteran Chetnik leader Kosta Pećanac to prevent the outbreak. However, the Bulgarians started conscripting Serbs, and hundreds of men joined the Chetnik detachments. This resulted in the 1917 Toplica Uprising under the leadership of Kosta Vojinović, which Pećanac eventually joined. Successful at first, the uprising was eventually put down by the Bulgarians and Austro-Hungarians, and bloody reprisals against the civilian population followed. Pećanac then used Chetniks for sabotage and raids against the Bulgarian occupation troops, then infiltrated the Austro-Hungarian occupied zone. Just prior to the end of the war, the Chetnik detachments were dissolved, with some sent home and others absorbed by the rest of the army. The Kingdom of Serbs, Croats and Slovenes was created with the merger of Serbia, Montenegro and the South Slav-inhabited areas of Austria-Hungary on 1 December 1918, in the immediate aftermath of the war.