'Reinhard' (i) was the German undertaking to exterminate all Polish Jews as the first phase of the planned extermination of all Jews in German-controlled Europe (November 1941/November 1943).

The Undertaking marked the beginning of the most deadly phase of the 'Holocaust', namely the use of special extermination camps. During the operation, as many as 2 million people, almost all of them Jews, were murdered in Belżec, Sobibor, Treblinka and Maidanek.

It is hypothesised that the operation was named in memory of SS-Obergruppenführer und General der Waffen-SS Reinhard Heydrich, the creator and co-ordinator of the Endlösung der Judenfrage (Final Solution of the Jewish Question), that is the extermination of the Jews living in the European countries occupied by the German forces in World War II.

After plans for the 'final solution' had been established at the Wannsee conference, Heydrich was attacked by members of the Czechoslovak resistance forces in 'Anthropoid' on 27 May 1942 and died eight days later. This hypothesis has been disputed by some, who argue that since the more mainstream designation of the operation was 'Reinhardt' (with a 't' after the 'd'), it could not have been named after Heydrich, and therefore argue that it was named after State Secretary of Finance Fritz Reinhardt. But in many official documents Heydrich’s name was written Reinhardt.

'Reinhard' was devised as a more effective method of mass murder which was also more 'humane', though only for those who conducted the killings as mass executions with firearms had revealed a decidedly adverse effect on the morale of the SS units involved. So 'Reinhard' used poisonous gas to mechanise the act of killing. It was the beginning of an industrialised mass murder unlike any previously known.

Starting from 1 November 1941, three extermination camps were constructed to cope with the population of adjacent ghettos and other victims from surrounding areas: first Belżec, then Sobibor and finally Treblinka, with Majdanek added in some accounts. These camps were located in the extreme east of Poland near the borders with Belorussia and Ukraine. The camps were located near main railway lines to facilitate prisoner transport, and in sparsely populated areas to preserve secrecy. As a guise, the victims were told that they were being transported east for resettlement and work.

Between 1.7 and 2 million people were exterminated in these camps, mostly by suffocation or poisoning by petrol engine exhaust gases. The head of 'Reinhard' was SS-Brigadeführer Odilo Globocnik, the SS and police chief of the Lublin district), appointed by Reichsführer-SS Heinrich Himmler. SS-Sturmbannführer Hermann Höfle, heading the Hauptabteilung 'Reinhard' administration, was responsible for personnel and the organisation of deportations, extermination camps and collection of the victims' valuables. Polizei-Kriminalkommissar Christian Wirth and his staff of the 'Aktion T4' euthanasia programme comprised the core of the execution staff in the extermination camps in eastern Poland. The camp guards were mostly Soviet POWs, Ukrainian volunteers and Volksdeutsche who were trained at the Trawniki camp near Lublin.

The headquarters of 'Reinhard' were located in Lublin, and the construction department in Zamosc. The clothes and belongings of the victims were stored and sorted in Lublin, at the unused hangars of Lublin airfield. The extermination process in Belżec, Sobibor and Treblinka was similar to the method used in the six euthanasia killing centres in Germany and Austria, but greatly enlarged in scale for killing whole transports of people at a time.

On arrival, victims handed over their valuables, which became property of the Reichsbank. They then undressed, and their clothes were searched for jewellery and other valuables. Those to be killed were next marched into the gas chamber and packed tightly to minimise the fresh air left to them. Carbon monoxide gas was then discharged through gas pipes, killing the occupants. The corpses were cremated after any gold dental fillings had been removed.

The mass murder was carefully tracked and documented. For example, the intercepted 'Höfle telegram' sent by Höfle on 11 January 1943 to SS-Obersturmbannführer Adolf Eichmann in Berlin listed 1,274,166 Jews killed in the four camps of 'Reinhard' during 1942 alone, breaking down the number of Jews killed by week by camp.

The structure of all camps was nearly identical. From the reception area with ramp and undressing barracks, the Jews entered a narrow, camouflaged path (called sluice or tube) to the extermination area with gas chambers, pits and cremation grids. The SS personnel and 'Trawniki men' (auxiliary police guards first trained at the Trawniki concentration camp) were housed in a separate area. Barbed wire fences, partially camouflaged with pine branches, surrounded the camp and separated the different parts. Unlike Auschwitz, no electric fences were used. Wooden watchtowers guarded the camp.

Some 2 million Jews lost their lives in Belżec, Sobibor, Treblinka and Majdanek in the course of 'Reinhard', and property valued at about 178,045,960 Reichsmarks was stolen. This money went not only to German authorities, but also to single individuals such as SS and police men, camp guards, non-Jewish inhabitants of towns and villages with ghettos or adjacent camps.

'Reinhard' ended in November 1943. After their work in the German concentration camps established in Poland, most of the staff was sent to northern Italy for actions against remaining Jews and partisans. Many of the perpetrators turned up again in the concentration camp of San Sabba near Trieste. The group disintegrated after the surrender of the German armed forces in Italy, only some of them eventually being brought to justice.