Built between 1927 and 1930 by the Otto Wagner's student Karl Ehn, the Karl Marx-Hof is a public housing complex (called "Gemeindebau" in Vienna), from the time of the social-democratic public housing programme during Austria's First Republic. With its 1382 Apartments and roughly 1,1 km in length, Marx-Hof is the world's longest contiguous public housing complex and probably Vienna's most famous "Gemeindebau". The permanent exhibit Red Vienna (in Wash House #2) is devoted to the history of Red Vienna, a period of one-party rule by the Social Democrats from 1919-1934. The non-exhibit parts of the building are still used as housing, so please respect the residents' privacy. Guided tours (German only) are offered on Sundays at 13:00, with no reservations required.

Karl-Marx-Hof (German: [karl maʁks hoːf]; lit. 'Karl Marx Court') is a Gemeindebau in Vienna, situated in Heiligenstadt, a neighbourhood of the 19th district of Vienna, Döbling.

At 1,100 metres (0.68 mi) in length and spanning four tram stops, Karl-Marx-Hof is one of the longest single residential buildings in the world.