THE EMERGENCE OF newly identified fungal pathogens and the reemergence of previously uncommon fungal diseases is primarily related to increases in the numbers of susceptible persons: people with HIV infection, bone marrow and organ transplant recipients, cancer patients being treated with chemotherapy, critically ill persons, and very low birth weight ( < or = 1500 g) infants. These immunocompromised populations are at risk for infection not only with opportunistic pathogens (for example, Pneumocystis, Candida, Cryptococcus, Trichosporon, Malassezia, Aspergillus, Penicillium marneffei, and numerous other moulds or yeasts) but also with fungal pathogens that usually infect otherwise healthy persons not previously exposed to endemic fungi (for example, Coccidioides immitis, Histoplasma capsulatum, and Blastomyces dermatitidis) and Sporothrix schenckii. Morbidity, mortality, and health care costs associated with fungal infections are high. Addressing the emergence of fungal diseases will require increased surveillance coupled with the availability of rapid, noninvasive diagnostic tests; monitoring the development of resistance to antifungal agents; and research focused on the understanding, prevention, and control of fungal infections.