It is difficult to overestimate the importance of the mould genera Penicillium and Aspergillus in the natural world and in the affairs of man. These are large and ub1quitous genera; several species are beneficial to mankind because of their role in food fermentation or the production of useful antibiotics and enzymes. Others are important as agents of food spoilage. In recent years, these moulds have ga1ned attention because a number of species produce toxic metabo lites. The great impact of mycotoxicology on food problems, particularly in developing countries has generated a vast body of research. The ecological roles of Aspergillus and Penicil lium species in nature is also of importance. For the past several decades, the taxonomic treatments of Raper and Thorn (1949) and Raper and Fennell (1965) ~lere the only standard descciptive monographs of Penicillium and Asper gillus. However, in practice, the identification of these economically important moulds was problematic and caused con siderable confusion in the literature and subsequently in applied research. In the past ten years, several laboratories have carried out taxonomic research resulting in a number of valuable publications. However, the differences in methodo logy and species concepts made comparison between the diffe rent classifications difficult. This confusion in the taxo nomy and nomenclature of common species is a great handicap to applied and industrial research.