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".

Reichsführer-SS was a special title and rank that existed between the years of 1925 and 1945 for the commander of the Schutzstaffel (SS). Reichsführer-SS was a title from 1925 to 1933, and from 1934 to 1945 it was the highest rank of the SS. The longest-serving and most noteworthy office holder was Heinrich Himmler.