Many of the prisoners died in the concentration camps due to deliberate maltreatment, disease, starvation, and overwork, or they were executed as unfit for labor. Prisoners were transported in inhumane conditions by rail freight cars, in which many died before reaching their final destination. The prisoners were confined in the boxcars for days or even weeks, with little or no 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 inmates perished because of harsh conditions or they were executed.

Auschwitz II (or "Birkenau") was completed in early 1942. Birkenau was built approximately 1.9 miles (3 km) away from Auschwitz I and was the real killing center of the Auschwitz death camp. It was in Birkenau where the dreaded selections were carried out on the ramp and where the sophisticated and camouflaged gas chambers laid in waiting. Birkenau, much larger than Auschwitz I, housed the most prisoners and included areas for women and Gypsies.

An inmate's first encounter with the camp, if they were being registered and not sent straight to the gas chamber, would be at the prisoner reception centre, where they were tattooed, shaved, disinfected, and given their striped prison uniform. Built between 1942 and 1944, the center contained a bathhouse, laundry, and 19 gas chambers for delousing clothes. Debórah Dwork and Robert Jan van Pelt write that inmates would then leave this area via a porch that faced the gate with the Arbeit macht frei sign. The prisoner reception center of Auschwitz I became the visitor reception center of the Auschwitz-Birkenau Memorial and Museum.[20]
Since the Nazis extended the Rassenschande ("race defilement") law to all foreigners at the beginning of the war,[139] pamphlets were issued to German women which ordered them to avoid sexual relations with foreign workers who were brought to Germany and the pamphlets also ordered German women to view these same foreign workers as a danger to their blood.[190] Although the law was applicable to both genders, German women were punished more severely for having sexual relations with foreign forced labourers in Germany.[191] The Nazis issued the Polish decrees on 8 March 1940 which contained regulations concerning the Polish forced labourers (Zivilarbeiter) who were brought to Germany during World War II. One of the regulations stated that any Pole "who has sexual relations with a German man or woman, or approaches them in any other improper manner, will be punished by death".[192]
The six extermination camps were established within a very short time. From December 1941 to December 1942 Chelmno, Belzec, Treblinka, Sobibor, Auschwitz-Birkenau and Majdanek all became operational. These sites were chosen because they were all situated near railway lines, in quiet rural areas of “far away” Poland, outside the spotlight of German and international public opinions.

The Allied powers organised war crimes trials, beginning with the Nuremberg trials, held from November 1945 to October 1946, of 23 top Nazi officials. They were charged with four counts—conspiracy to commit crimes, crimes against peace, war crimes and crimes against humanity—in violation of international laws governing warfare.[483] All but three of the defendants were found guilty and twelve were sentenced to death.[484] Twelve Subsequent Nuremberg trials of 184 defendants were held between 1946 and 1949.[483] Between 1946 and 1949, the Allies investigated 3,887 cases, of which 489 were brought to trial. The result was convictions of 1,426 people; 297 of these were sentenced to death and 279 to life in prison, with the remainder receiving lesser sentences. About 65 percent of the death sentences were carried out.[485] Poland was more active than other nations in investigating war crimes, for example prosecuting 673 of the total 789 Auschwitz staff brought to trial.[486]
Auschwitz-Birkenau, Nazi Germany's largest concentration and extermination camp facility, was located nearby the provincial Polish town of Oshwiecim in Galacia, and was established by order of Reichsfuhrer SS Heinrich Himmler on 27 April 1940. Private diaries of Goebbels and Himmler unearthed from the secret Soviet archives show that Adolf Hitler personally ordered the mass extermination of the Jews during a meeting of Nazi German regional governors in the chancellery. As Goebbels wrote "With regards to the Jewish question, the Fuhrer decided to make a clean sweep ..."
Heinrich Himmler's Schutzstaffel (SS) took full control of the police and the concentration camps throughout Germany in 1934–35.[5] Himmler expanded the role of the camps to hold so-called "racially undesirable elements", such as Jews, Gypsies/Romanis/Sintis, Serbs, Poles, disabled people, and criminals.[6][7][8] The number of people in the camps, which had fallen to 7,500, grew again to 21,000 by the start of World War II[9] and peaked at 715,000 in January 1945.[10]