Auschwitz–Birkenau, located in the town of Oświęcim, was the largest slave labor and extermination center in Nazi-occupied Poland. It was designated by Heinrich Himmler, Germany's Minister of the Interior, as the locus of the "final solution of the Jewish question in Europe." From spring 1942 until the fall of 1944, transport trains delivered Jews to the camp's gas chambers from all over Nazi-occupied Europe.  The camp's first commandant, Rudolf Höss, later testified at the Nuremberg Trials that up to three million people may have died there.  The figure has been since revised to 1.1 million, 90 percent of them being Jews.  Others deported to Auschwitz included 150,000 Poles, 23,000 Roma and Sinti, 15,000 Soviet prisoners of war, and tens of thousands of other nationalities. Those not killed in the gas chambers died of starvation, forced labor, disease, individual executions, or purported medical experiments.

On September 3, 1941, deputy camp commandant SS-Hauptsturmführer Fritzsch experimented on 600 Russian POWs and 250 Polish inmates by gassing them with Zyklon B, a highly lethal cyanide-based pesticide. This paved the way for the use of Zyklon B as an instrument for extermination at Auschwitz, and a gas chamber and crematorium were constructed. This gas chamber operated from 1941 to 1944, during which time some 60,000 people were killed; before it was converted into an air-raid shelter.

Construction on the nearby extermination camp, Auschwitz II-Birkenau, began in October 1941 to ease congestion at the main camp. It was larger than the original Auschwitz camp, and more people passed through its gates. It was designed to hold several categories of prisoners, and to function as an extermination camp in the context of Heinrich Himmler's preparations for the Final Solution of the Jewish Question. The first gas chamber at Birkenau, "The Little Red House," was a brick cottage converted into a gassing facility. It was operational by March 1942. A second brick cottage, "The Little White House," was similarly converted some weeks later.

In early 1943, the Nazis decided to increase greatly the gassing capacity of Birkenau. Crematorium II, originally designed as a mortuary, with morgues in the basement and ground-level furnaces, was converted into a killing factory by placing a gas-tight door on the morgues and adding vents for Zyklon B and ventilation equipment to remove the gas. It came online in March. Crematorium III was built using the same design. Crematoria IV and V, designed from the start as gassing centers, were also constructed that spring. By June 1943 all four crematoria were up. Most victims were killed during a period afterwards.

By July 1942, the SS were conducting the infamous "selections," in which incoming Jews were divided into those deemed able to work, who were sent to the right and admitted into the camp, and those who were sent to the left and immediately gassed.   Prisoners were transported from all over German-occupied Europe by rail, arriving in daily convoys. The group selected to die, about three-quarters of the total, included almost all children, women with children, all the elderly, and all those who appeared by a brief and superficial inspection by an SS doctor not to be completely fit. Auschwitz II-Birkenau claimed more victims than any other German extermination camp, despite coming into use after all the others.

SS officers told the victims they were to take a shower and undergo delousing. The victims would undress in an outer chamber and walk into the gas chamber, which was disguised as a shower facility, complete with dummy shower heads. After the doors were shut, SS men would dump in the cyanide pellets via holes in the roof or windows on the side. In Auschwitz II-Birkenau, more than 20,000 people could be gassed and cremated each day. The Nazis used a cyanide gas produced from Zyklon B pellets, manufactured by two companies who had acquired licensing rights to the patent held by IG Farben.

Prisoners known as Sonderkommandos removed gold teeth from the corpses of gas chamber victims; the gold was melted down and sent back to the Third Reich. The belongings of the arrivals were seized by the SS and sorted in an area of the camp called "Canada," so-called because Canada was seen as a land of plenty. Many of the SS at the camp enriched themselves by pilfering the confiscated property.

The gas chambers worked to their fullest capacity from April-July 1944, during the massacre of Hungary's Jews. Hungary was an ally of Germany during the war, but it had resisted turning over its Jews to the Germans until Germany invaded in March 1944. From April until July 9, 1944, 475,000 Hungarian Jews, half of the pre-war population, were deported to Auschwitz, at a rate of 12,000 a day for a considerable part of that period. The incoming volume was so great that the SS resorted to burning corpses in open-air pits as well as in the crematoria.

Those not selected upon arrival for death in the gas chambers were sent to surrounding slave labor camps and forced to work in German factories.  The largest of the Auschwitz work camps was Auschwitz III-Monowitz. Starting operations in May 1942, it was associated with the synthetic rubber and liquid fuel plant Buna-Werke owned by IG Farben. 11,000 slave laborers worked at Monowitz. Seven thousand inmates worked at various chemical plants. 8,000 worked in mines. Approximately 40,000 prisoners worked in slave labor camps at Auschwitz or nearby, under appalling conditions. In regular intervals, doctors from Auschwitz II would visit the work camps and select the weak and sick for the gas chambers of Birkenau.

The last selection took place on October 30, 1944. The next month, Heinrich Himmler ordered the crematoria destroyed before the Red Army reached the camp. The gas chambers of Birkenau were blown up by the SS in January 1945 in an attempt to hide the German crimes from the advancing Soviet troops. The SS command sent orders on January 17, 1945 calling for the execution of all prisoners remaining in the camp, but in the chaos of the Nazi retreat the order was never carried out. On January 17, 1945, Nazi personnel started to evacuate the facility. Nearly 60,000 prisoners were forced on a death march. Those too weak or sick to walk were left behind. These remaining 7,500 prisoners were liberated by the 322nd Rifle Division of the Red Army on January 27, 1945. Approximately 20,000 Auschwitz prisoners made it to Bergen-Belsen concentration camp in Germany, where they were liberated by the British in April 1945.