Karel Appel is best known for his contribution to CoBrA (from Copenhagen, Brussels, Amsterdam) that he joined in 1948, together with the Dutch artists Corneille, Constant, and Jan Nieuwenhuys (see also Aart Kemink), and with the Belgian poet Christian Dotremont. The new art of the CoBrA group was not popular in the Netherlands, but it found a warm and broad welcome in Denmark. By 1949, Danish artists had already started to make spontaneous art and one of their sources of inspiration was Danish and Nordic mythology. It was also in Denmark that the CoBrA artists started cooperating by collectively painting the insides of houses, which encouraged and intensified the exchange of the typical ‘childish’ and spontaneous picture language used by the CoBrA group. Appel used this very intensively; his 1949 fresco ‘Questioning Children’ in Amsterdam City Hall caused controversy and was covered up for ten years.
As a result of this controversy and other negative Dutch reactions to CoBrA, Appel moved to Paris in 1950 and developed his international reputation by travelling to Mexico, the USA, Yugoslavia, and Brazil. He also lived in New York City and Florence. His first American gallery exhibition took place in 1954 at the Martha Jackson Gallery. The following year his painting Child and Beast II (1951) was included in the influential exhibition, The New Decade at the MoMA, New York which featured the work of twenty-two European painters and sculptors including newcomers like Francis Bacon, Jean Dubuffet, and Pierre Soulages. He is particularly noted for his mural work. After 1990 he became much more popular in the Netherlands; he had several big shows in Amsterdam and Bruxelles, organized by director Rudy Fuchs. Also, the CoBrA-museum in Amstelveen organized several shows featuring his work. He became probably the most reknown Dutch CoBrA artist.