Sicherheitsdienst, full title Sicherheitsdienst des Reichsführers-SS, or SD, was the intelligence agency of the SS and the Nazi Party in Nazi Germany. Established in 1931, the SD was the first Nazi intelligence organization and the Gestapo was considered its sister organization through the integration of SS members and operational procedures. The SD was administered as an independent SS office between 1933 and 1939. That year, the SD was transferred over to the Reich Security Main Office, as one of its seven departments. Its first director, Reinhard Heydrich, intended for the SD to bring every single individual within the Third Reich's reach under "continuous supervision".

Nazi Germany, officially known as the German Reich and later the Greater German Reich, is a term used to describe the German state between 1933 and 1945, when Adolf Hitler and the Nazi Party controlled the country, transforming it into a totalitarian dictatorship. The Third Reich, meaning "Third Realm" or "Third Empire", referred to the Nazi claim that Nazi Germany was the successor to the earlier Holy Roman Empire (800–1806) and German Empire (1871–1918). The Third Reich, which the Nazis referred to as the Thousand-Year Reich, ended in May 1945, after only 12 years, when the Allies defeated Germany and entered the capital, Berlin, ending World War II in Europe.