A revaluation is made of the genus Chaetomium which comprises about 300 species, and relatives classified in several genera of the Chaetomiaceae, Melanosporaceae and Microascaceae. The classification is based on the size, shape and symmetry of the ascospores, the number of the germ pores, the shape of the asci (clavate, obovate, fusiform or cylindrical) and the size, shape, structure and ornamentation of the ascomata.