Clinical and Laboratory Standards Institute standard M27—Reference Method for Broth Dilution Antifungal Susceptibility Testing of Yeasts describes a method for testing the susceptibility to antifungal agents of yeasts that cause invasive fungal infections, including Candida spp. and Cryptococcus neoformans. Selection and preparation of antifungal agents, implementation and interpretation of test procedures, and the purpose and implementation of QC procedures are discussed. A careful examination of the responsibilities of the manufacturer and the user in QC is also presented.