Reinhard Tristan Eugen Heydrich (; German: [ˈʁaɪnhaʁt ˈtʁɪstan ˈʔɔʏɡn̩ ˈhaɪdʁɪç] (listen); 7 March 1904 – 4 June 1942) was a high-ranking German SS and police official during the Nazi era and a principal architect of the Holocaust. He was chief of the Reich Security Main Office (including the Gestapo, Kripo, and SD). He was also Stellvertretender Reichsprotektor (Deputy/Acting Reich-Protector) of Bohemia and Moravia. He served as president of the International Criminal Police Commission (ICPC, later known as Interpol) and chaired the January 1942 Wannsee Conference which formalised plans for the "Final Solution to the Jewish Question"—the deportation and genocide of all Jews in German-occupied Europe.

Many historians regard Heydrich as the darkest figure within the Nazi regime; Adolf Hitler described him as "the man with the iron heart". He was the founding head of the Sicherheitsdienst (Security Service, SD), an intelligence organisation charged with seeking out and neutralising resistance to the Nazi Party via arrests, deportations, and murders. He helped organise Kristallnacht, a series of coordinated attacks against Jews throughout Nazi Germany and parts of Austria on 9–10 November 1938. The attacks were carried out by SA stormtroopers and civilians and presaged the Holocaust. Upon his arrival in Prague, Heydrich sought to eliminate opposition to the Nazi occupation by suppressing Czech culture and deporting and executing members of the Czech resistance. He was directly responsible for the Einsatzgruppen, the special task forces that travelled in the wake of the German armies and murdered more than two million people by mass shooting and gassing, including 1.3 million Jews.

Heydrich was mortally wounded in Prague on 27 May 1942 as a result of Operation Anthropoid. He was ambushed by a team of Czech and Slovak soldiers who had been sent by the Czechoslovak government-in-exile to kill the Reich-Protector; the team was trained by the British Special Operations Executive. Heydrich died from his injuries a week later. Nazi intelligence falsely linked the Czech and Slovak soldiers and resistance partisans to the villages of Lidice and Ležáky. Both villages were razed; the men and boys age 14 and above were shot, and most of the women and children were deported and murdered in Nazi concentration camps.