From 1933 to 1945, Nazi Germany operated more than a thousand camps described as concentration camps (German: Konzentrationslager), including subcamps on its own territory and in parts of German-occupied Europe.

The first camps were established in March 1933 immediately after Adolf Hitler became Chancellor of Germany. Following the 1934 purge of the SA, the concentration camps were run exclusively by the SS via the Concentration Camps Inspectorate and later the SS Main Economic and Administrative Office. Initially, most prisoners were members of the Communist Party of Germany, but as time went on different groups were arrested, including "habitual criminals", "asocials", and Jews. After the beginning of World War II, people from German-occupied Europe were imprisoned in the concentration camps. About 1.65 million people were registered prisoners in the camps, of whom about a million died during their imprisonment. Most of the fatalities occurred during the second half of World War II, including at least a third of the 700,000 prisoners who were registered as of January 1945. Following Allied military victories, the camps were gradually liberated in 1944 and 1945, although hundreds of thousands of prisoners died in death marches.

Museums commemorating the victims of the Nazi regime have been established at many of the former camps, and the Nazi concentration camp system is universally known for violence, terror, and mass murder.

Concentration camps are conventionally held to have been invented by the British during the Second Boer War, but historian Dan Stone argues that there were precedents in other countries and that camps were "the logical extension of phenomena that had long characterized colonial rule". Although the word "concentration camp" has acquired the connotation of murder because of the Nazi concentration camps, the British camps in South Africa did not involve systematic murder. The German Empire also established concentration camps during the Herero and Namaqua genocide (1904–1907); the death rate of these camps was 45 per cent, twice that of the British camps.

During the First World War, eight to nine million prisoners of war were held in prisoner-of-war camps, some of them at locations which were later the sites of Nazi camps, such as Theresienstadt and Mauthausen. Many prisoners held by Germany died as a result of intentional withholding of food and dangerous working conditions in violation of the 1907 Hague Convention. In countries including France, Belgium, Italy, Austria-Hungary, and Germany, civilians deemed to be of "enemy origin" were denaturalized. Hundreds of thousands were interned and subject to forced labor in harsh conditions. During the Armenian genocide perpetrated by the Ottoman Empire, Ottoman Armenians were held in camps during their deportation into the Syrian Desert. In postwar Germany, "unwanted foreigners" – mainly Eastern European Jews – were confined at Cottbus-Sielow and Stargard.

On 30 January 1933, Adolf Hitler became chancellor of Germany after striking a backroom deal with the previous chancellor, Franz von Papen. The Nazis had no plan for concentration camps prior to their seizure of power. The concentration camp system arose in the following months due to the desire to suppress tens of thousands of Nazi opponents in Germany. The Reichstag fire in February 1933 was the pretext for mass arrests. The Reichstag Fire Decree eliminated the right to personal freedom enshrined in the Weimar Constitution and provided a legal basis for detention without trial. The first camp was Nohra, established on 3 March 1933 in a school.

The number of prisoners in 1933–1934 is difficult to determine; historian Jane Caplan estimated it at 50,000, with arrests perhaps exceeding 100,000. Eighty per cent of prisoners were members of the Communist Party of Germany and ten per cent members of the Social Democratic Party of Germany. Many prisoners were released in late 1933, and after a Christmas amnesty, there were only a few dozen camps left. About 70 camps were established in 1933, in any convenient structure that could hold prisoners, including vacant factories, prisons, country estates, schools, workhouses, and castles. There was no national system; camps were operated by local police, SS, and SA, state interior ministries, or a combination of the above. The early camps in 1933–1934 were heterogeneous and fundamentally differed from the post-1935 camps in organization, conditions, and the groups imprisoned.

On 26 June 1933, Himmler appointed Theodor Eicke the second commandant of Dachau, which became the model followed by other camps. Eicke drafted the Disciplinary and Penal Code, a manual which specified draconian punishments for disobedient prisoners. He also created a system of prisoner functionaries, which later developed into the camp elders, block elders, and kapo of later camps. In May 1934, Lichtenburg was taken over by the SS from the Prussian bureaucracy, marking the beginning of a transition set in motion by Heinrich Himmler, then chief of the Gestapo (secret police). Following the purge of the SA on 30 June 1934, in which Eicke took a leading role, the remaining SA-run camps were taken over by the SS. In December 1934, Eicke was appointed the first inspector of the Concentration Camps Inspectorate (IKL); only camps managed by the IKL were designated "concentration camps".