Clinical and Laboratory Standards Institute standard M38—Reference Method for Broth Dilution Antifungal Susceptibility Testing of Filamentous Fungi describes a method for testing the susceptibility to antifungal agents of filamentous fungi (nondermatophyte and dermatophyte moulds) that cause invasive and/or cutaneous fungal infections. Antifungal agent selection, preparation of antifungal stock solutions and dilutions for testing, test procedure implementation and interpretation, and the purpose and implementation of QC procedures are discussed. A careful examination of manufacturer and user responsibilities in QC is also presented. In addition, a brief discussion regarding newly defined epidemiological cutoff values for certain Aspergillus spp. and species complexes are included.