Stefan or Stephan Cohn-Vossen (28 May 1902 – 25 June 1936) was a mathematician, now best known for his collaboration with David Hilbert on the 1932 book Anschauliche Geometrie, translated into English as Geometry and the Imagination. The Cohn-Vossen transformation is also named for him.He was born in Breslau (then a city in the German Empire; now Wrocław in Poland).