File:Infamous train track to carry prisoners to gas chambers for execution - Auschwitz II-Birkenau (9307428048).jpg

Construction on Auschwitz II-Birkenau, the extermination camp, began in October 1941 to ease congestion at the main camp. It was larger than Auschwitz I, and more people passed through its gates than through Auschwitz I. 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 extermination of the Jews. The first gas chamber at Birkenau was "The Little Red House", a brick cottage converted into a gassing facility by tearing out the inside and bricking up the walls. It was operational by March 1942. A second brick cottage, "The Little White House, was similarly converted some weeks later.

The Nazis had committed themselves to the final solution no later than January 20, 1942, the date of the Wannsee Conference.

British historian Laurence Rees writes, that Höss may have misremembered the year Himmler said this. Himmler did indeed visit Höss in the summer of 1941, but there is no evidence that the final solution had been planned at this stage. Rees writes that the meeting predates the killings of Jewish men by the Einsatzgruppen in the East and the expansion of the killings in July 1941. It also predates the Wannsee Conference. Rees speculates that the conversation with Himmler was most likely in the summer of 1942. The first gassings, using an industrial gas derived from prussic acid and known by the brand name Zyklon-B, were carried out at Auschwitz in September 1941.

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 went into operation 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 operational. Most of the victims were killed during the period afterwards. The camp was staffed partly by prisoners, some of whom were selected to be kapos (orderlies, most of whom were convicts) and sonderkommandos (workers at the crematoria). The kapos were responsible for keeping order in the barrack huts; the sonderkommandos prepared new arrivals for gassing (ordering them to remove their clothing and surrender their personal possessions) and transferred corpses from the gas chambers to the furnaces, having first pulled out any gold that the victims might have had in their teeth. Members of these groups were killed periodically. The kapos and sonderkommandos were supervised by members of the SS; altogether 6,000 SS members worked at Auschwitz.