Aspergillus species are the most common causes of invasive mold infections in immunocompromised patients. The introduction of new antifungal agents, and recent reports of resistance emerging during treatment of aspergillus infections, have highlighted the need for standardized methods of antifungal drug susceptibility testing for filamentous fungi. This review describes the methods that are now being developed for the in vitro testing of Aspergillus species, and the results of attempts to correlate in vitro findings with in vivo outcome. The mechanisms and clinical importance of resistance to the different agents used in the treatment of human aspergillosis are discussed. Copyright 1999 Harcourt Publishers Ltd.