From 1940 to 1943, during the German occupation, Appel studied at the Rijksakademie van Beeldende Kunsten in Amsterdam, and it was there he met the young painter Corneille and, some years later, Constant; they became close friends for years. His parents opposed his choice to become an artist, leading him to leave home; this was also necessary because he needed to hide from the German police so that he would not be picked up and sent to Germany to work in the weapons industry. Appel had his first show in Groningen in 1946. In 1949 he participated with the other CoBrA artists in the Stedelijk Museum Amsterdam; this generated a huge scandal and many objections in the press and public. He was influenced by Pablo Picasso, Henri Matisse, and the French brute-art artist Jean Dubuffet. In 1947 he started sculpting with all kinds of used materials (in the technique of assemblage) and painted them in bright colors: white, red, yellow, blue, and black. He joined the Experimentele Groep in Holland [nl] together with the young Dutch painters Anton Rooskens, Theo Wolvecamp, and Jan Nieuwenhuys. Later the Belgian writer Hugo Claus joined the group.