Appel's career was animated by a single sustained refusal, to paint like a trained European modernist after the Second World War, and the body of work that refusal produced, raw, primary-colored, and unembarrassedly emotional, constitutes one of the most distinctive contributions to twentieth-century European painting.
Appel came to art in occupied Amsterdam. His early painting at fourteen was self-directed, and his subsequent training during the war years gave him a thorough grounding in academic technique that he would soon set against the foundations of his mature practice. His early influences ranged across the leading figures of European modernism toward the figures who would shape the postwar reaction against academic painting: the brut tradition, the art of children, and the work of the institutionalized and the untrained.
The founding of CoBrA was the decisive moment of his career. The group's commitment to a childlike, spontaneous, primitive painting practice, drawn from the visual languages of children, folk artists, and the mentally ill, rejected the rationalism that Appel and his collaborators identified with the European political and intellectual culture that had produced the war. The work was deliberately raw, color-saturated, gestural, and figurative, and it found a hostile reception in the Netherlands but a much more sympathetic audience in Denmark, where the group's early exhibitions were warmly received. CoBrA itself dissolved in 1951, but the painters who had organized it continued in their broadly shared idiom for decades afterward.
Appel emerged from the CoBrA years as the movement's most internationally visible figure. The major exhibitions of the early 1950s and his eventual move to Paris gave him a platform on the broader European stage, and the institutional recognition that followed confirmed his standing in his own country. The bright, scrubbed, near-primary palette and densely worked surfaces of the paintings he produced through the 1950s and 1960s, populated by snarling figures, wide-eyed faces, animals, and quasi-figurative shapes, became the recognizable signature of his work.
His practice extended in every direction available to a postwar artist. He produced painted assemblage sculpture from used and salvaged materials, ceramic work, set and costume designs for theatre and ballet, illustrated books, and a substantial body of poetry. His later decades saw him working between studios in Paris, New York, and Tuscany, and the work continued to develop without ever abandoning the original CoBrA commitment to spontaneity and emotional directness.
His position today is settled at the highest level of postwar European painting. He is the most internationally recognized of the CoBrA painters, his work is held by every major museum of twentieth-century art, and his refusal to paint like a trained European modernist after the war has continued to be read as one of the defining gestures of European postwar Expressionism.