Reichsführer-SS (German: [ˈʁaɪçsˌfyːʁɐ ˈɛs ˈɛs]) 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 by far most noteworthy Reichsführer-SS was Heinrich Himmler.
Reichsführer-SS was both a title and a rank. The title of Reichsführer was first created in 1926 by the second commander of the SS, Joseph Berchtold. Berchtold's predecessor, Julius Schreck, never referred to himself as Reichsführer, but the title was retroactively applied to him in later years. In 1929, Heinrich Himmler became Reichsführer-SS and referred to himself by his title instead of his regular SS rank of Obergruppenführer. This set the precedent for the commander of the SS to be called Reichsführer-SS.
Prior to the Night of the Long Knives, the SS was an elite corps of the Sturmabteilung (SA or storm troopers), and the Reichsführer-SS was subordinate to the SA's operating head, the Stabschef. On 20 July 1934, as part of the purge of the SA, the SS was made an independent branch of the Nazi Party, responsible only to Hitler. From that point on, the title of Reichsführer-SS became an actual rank, and in fact the highest rank of the SS. In this position, Himmler was on paper the equivalent of a Generalfeldmarschall in the German Army. As Himmler's position and authority grew in Nazi Germany, so did his rank in a "de facto" sense. Further, there was never more than one Reichsführer-SS at any one time, with Himmler holding the position as his personal title from 1929 (becoming his actual rank in 1934) until April 1945.