Charles Ehresmann (19 April 1905 – 22 September 1979) was a German-born French mathematician who worked in differential topology and category theory.

In 1939 Ehresmann became a lecturer at the University of Strasbourg, but one year later the whole faculty was evacuated to Clermont-Ferrand due to the German occupation of France.

[1][2] In the first part of his career Ehresmann introduced many new mathematical objects in differential geometry and topology, which gave rise to entire new fields, often developed later by his students.

[10][11] He developed the concept of fiber bundle, and the related notions of Ehresmann connection and solder form,[12][13][14][15] building on the works by Herbert Seifert[16] and Hassler Whitney[17] in the 1930s.

Norman Steenrod was working in the same direction from a topological point of view,[18] but Ehresmann, influenced by Cartan's ideas, was particularly interested in differentiable (smooth) fiber bundles, and in the differential-geometric aspects of these.

[20] In order to obtain a more conceptual understanding of completely integrable systems of partial differential equations, in 1944 Ehresmann inaugurated the theory of foliations,[21][22] which will be later developed by his student Georges Reeb.