The number of fungal species reported to cause disease in man is increasing rapidly. Very few of these fungi are capable of infecting a normal host. Important progress has been achieved in an understanding of fungal pathogenicity including the mechanisms of adherence to host tissues, penetration of tissues, multiplication within the host, and the interaction of fungal cells with host effector cells. In addition to the increase in infections by opportunistic and pathogenic fungi in compromised patients, caused by Candida spp., Aspergillus spp., Cryptococcus neoformans, Histoplasma capsulatum and Coccidioides immitis, many fungi that occur as saprophytes in the environment and which had previously been considered to be nonpathogenic are now being encountered as causes of human infection. The advent of these unusual infections has led to reappraisal of the diagnostic tests involved in the investigation of fungal infections and has had important implications for the choice of treatment. Many of these fungi have a similar tissue appearance and the organisms must be isolated and identified to ensure that the most appropriate treatment is given.