These early prisons (usually basements and storehouses) were eventually consolidated into full-blown, centrally run camps outside the cities and somewhat removed from the public eye. By 1939, six large concentration camps had been established: Dachau (1933), Sachsenhausen (1936), Buchenwald (1937), Flossenbürg (1938), Mauthausen (1939), and Ravensbrück (1939).
In 1938, the SS began to use the camps as a source of forced labor for profit-making ventures. Many German companies used forced laborers from them, especially during the war (see Forced labor in Germany during World War II).
Additionally, historians speculate that the Nazi regime used abandoned castles and similar existing structures to lock up the undesirable elements of society. The elderly, mentally ill, and handicapped were often confined in these makeshift camps where they were starved or gassed to death with diesel engine exhaust. The Final Solution was therefore initially tested on German citizens. (See Action T4, the Nazi program of racial hygiene.)
After 1939 with the beginning of the 2nd World War, concentration camps increasingly became places where the enemies of the Nazis were killed, enslaved, starved, and tortured. During the War concentration camps for “undesirables” spread throughout Europe. New camps were created near centers of dense “undesirable” populations, often focusing on areas with large communities of Jewish, Polish intelligentsia, Communists, or Roma. Most camps were located in the area of General Government in occupied Poland for a simple logistical reason: millions of Jews lived in Poland. It also allowed the Nazis to transport the German Jews outside of the German main territory. '''
Sometimes the concentration camps were used to hold important prisoners, such as the generals involved in the attempted assassination of Hitler; U-Boat Captain-turned-Lutheran pastor Martin Niemöller; and Admiral Wilhelm Canaris, who was interned at Flossenbürg on February 7, 1945, until he was hanged on April 9, shortly before the war’s end.
In most camps, prisoners were forced to wear identifying overalls with colored badges according to their categorization: red triangles for Communists and other political prisoners, green triangles for common criminals, pink for homosexual men, purple for Jehovah’s Witnesses, black for Gypsies and asocials, and yellow for Jews.
In these concentration camps, millions of prisoners died through mistreatment, disease, starvation, and overwork, or were executed as unfit for labour; though they were not extermination or death camps which started in 1942. These camps were located in occupied Poland and some territories of today's Belarus, on the territory of the General Government. More than three million Jews would die in them, primarily by poisonous gas, usually in gas chambers, although many prisoners were killed in mass shootings and by other means.
Prisoners were often transported under horrifying conditions using rail freight cars, in which many died before they reached their destination, and there were instances where only ten prisoners-to-be would be alive to come out of a cart packed with one hundred. The prisoners were confined to the rail cars, often for days or weeks, without food or water. Many died of dehydration in the intense heat of summer or froze to death in winter. Concentration camps also existed in Germany itself, and while they were not specifically designed for systematic extermination, many of their prisoners perished because of harsh conditions or execution.
In the early spring of 1941 the SS, along with doctors and officials of the T-4 Euthanasia Program, began killing selected concentration camp prisoners in “Operation 14f13.” The Inspectorate of the Concentration Camps categorized all files dealing with the death of prisoners as 14f, and those of prisoners sent to the T-4 gas chambers as 14f13. Under the language regulations of the SS, selected prisoners were designated for “special treatment (German: Sonderbehandlung) 14f13”. Prisoners were officially selected based on their medical condition; namely, those permanently unfit for labor due to illness. Unofficially, racial and eugenic criteria were used: Jews, the handicapped, and those with criminal or antisocial records were selected. For Jewish prisoners there was not even the pretense of a medical examination: the arrest record was listed as a physician’s “diagnosis”. In early 1943, as the need for labor increased and the gas chambers at Auschwitz became operational, Heinrich Himmler ordered the end of Operation 14f13.
After 1942, many small subcamps were set up near factories to provide forced labor. IG Farben established a synthetic rubber plant in 1942 at Auschwitz III (Monowitz), and other camps were set up next to airplane factories, coal mines, and rocket fuel plants. Conditions were brutal, and prisoners were often sent to the gas chambers or killed if they did not work fast enough.
Near the end of the war, the camps became sites for horrific medical experiments. Eugenics experiments, freezing prisoners to determine how exposure affected pilots, and experimental and lethal medicines were all tried at various camps. Female prisoners were routinely raped and degraded in the camps.
The camps were liberated by the Allies between 1943 and 1945, often too late to save the prisoners remaining. For example, when the UK entered Bergen-Belsen concentration camp in 1945, 60,000 prisoners were found alive, but 10,000 died within a week of liberation due to typhus and malnutrition.
The British intelligence service had information about the concentration camps, and in 1942 Jan Karski delivered a thorough eyewitness account to the government.
In Communist Poland, (Majdanek, Jaworzno, Potulice, Zgoda) and East Germany (Buchenwald, Sachsenhausen), German POWs, suspected Nazis and collaborators, anti-Communists and other political prisoners, as well as civilian members of German, Ukrainian and other ethnic minorities were held in some of the camps between 1945 and 1956. (See also: Soviet special camps)