Reichskommissariat (English: lit. Realm Commissariat; plural Reichskommissariate) is the German designation for a type of administrative office headed by a government official known as a Reichskommissar (English: Realm Commissioner). Although many different such offices existed primarily throughout the Imperial German and Nazi periods in a number of different fields (such as public infrastructure, spatial planning, ethnic cleansing, etc.) it is most commonly used to refer to the quasi-colonial territorial units established by Nazi Germany in several occupied countries during World War II. While officially located outside of the German Reich in a legal sense, these entities were directly controlled by their supreme civil authorities (the Reichskommissars), who ruled their assigned territories as German governors on behalf of and as direct representatives of Adolf Hitler.
The introduction of these territorial administrations served a number of purposes. Those established or planned to be established in Western and Northern Europe were in general envisioned as the transitional phases for the future incorporation of various Germanic countries outside of pre-war Germany into an expanded Nazi state. Their eastern counterparts served primarily colonialist and imperialist purposes, as sources of future Lebensraum for German settlement and the exploitation of natural resources.
Another contrast was the level of administrative overhaul implemented in these two types. As in most other territories conquered by the Germans, local administrators and bureaucrats were pressured to continue their regular day-to-day operations (especially at the middle and lower levels) albeit under German oversight. Throughout the war the Reichskommissariate in Western and Northern Europe simply retained the previously existing administrative structure however, while in the eastern ones completely new such structures were introduced.
All of these entities were nonetheless intended for eventual integration into a Greater Germanic Reich (Grossgermanisches Reich) encompassing the general area of Europe stretching from the North Sea to the Ural mountains, for which Germany was to form the basis.
Western and Northern Europe 
- Reichskommissariat Norwegen (German-occupied territories of Norway); 1940–1945.
- Reichskommissariat Niederlande (German-occupied Netherlands); 1940–1945.
- Reichskommissariat Belgien-Nordfrankreich (German-occupied Belgium and Nord-Pas-de-Calais); 1944. Annexed directly into the Greater German Reich in December 1944 as the new Reichsgaue of Flanders, Wallonia and Brussels.
Former Soviet territories 
In summer 1941, German Nazi-ideologist Alfred Rosenberg suggested that to facilitate the break-up of the Soviet Union and Russia as a geographical entity, conquered Soviet territory should be administered in the following five Reichskommissariate:
- Reichskommissariat Ostland (RKO) (The Baltic countries and Belarus, extended eastward by including some parts of Western Russia) 1941–1944/45.
- Reichskommissariat Ukraine (RKU) (Ukraine minus East Galicia, Romanian Transnistria and the Crimea, but extended eastward up to the Volga); 1941–1944.
- Reichskommissariat Moskowien (RKM) (Moscow metropolitan area and the rest of European Russia minus Karelia and the Kola peninsula, promised to Finland); never fully established. German military advance halted in 1941/42.
- Reichskommissariat Kaukasus (RKK) (Southern Russia and the Caucasus area); never fully established. German military advance halted in 1942/43.
- Reichskommissariat Turkestan (RKT) (Central Asian republics and territories); never established.
At Hitler's request the Turkestan project was shelved by Rosenberg for the immediate future, who was instead ordered to focus on Europe for the time-being. Central Asia was determined to be a future target for German expansion, as soon as its armies would be ready to move further east after the consolidation of present victories in Soviet Russia. The interest in part of the area of Germany's major Axis partner, the Empire of Japan (see Axis power negotiations on the division of Asia during World War II), could have become a topic of discussion regarding their own contemporaneous establishment of the Greater East Asia Co-Prosperity Sphere.
- Rich, Norman: Hitler's War Aims: The Nazi State and the Course of Expansion, p. 217. W. W. Norton & Company, New York 1974.
- Bohn, Robert: Die deutsche Herrschaft in den "germanischen" Ländern 1940-1945, p. 39. Steiner, 1997. 
- Gumkowski, Janusz; Leszczynski, Kazimierz: Poland Under Nazi Occupation. Polonia Pub. House, 1961. 
- Kay, Alex J: Exploitation, resettlement, Mass Murder: Political and Economic Planning for German Occupation Policy in the Soviet Union, 1940–1941. Berghahn Books, 2006. 
- Oversight of the planned territorial organization of the Reichskommissariate
- Dallin, Alexander: German rule in Russia 1941–1945: A Study of Occupation Policies. Westview press, 1981 
- Wasser, Bruno: Himmler's Raumplanung in Osten: Der Generalplan Ost in Polen 1940-1945, p. 51. Birkhäuser, 1993.