Normal oral flora comprises a diverse variety of microorganisms which includes eubacteria, archaebacteria, fungi, mycoplasmas and protozoa. Among these, fungi are classified as eukaryotes, and the most important one belong to the genus Candida. Human infections caused by Candida albicans and other related spp. range from the more common oral thrush to fatal, systemic super infections in patients who are afflicted with other debilitating diseases and in immunocompromised persons. There are more than 100,000 known fungi, but only few invade human tissues. These fungi are common saprophytes of soil and decaying vegetation, and person to person transmission is rare, but Candida is an exception. The main obstacles for these fungi to grow in man are the elevated temperature and cellular immunity. Humoral immunity does not seem to be important. Systemic mycoses are common in endemic areas, but usually resolve spontaneously without treatment. Most are opportunistic infections, involving immunosupressed persons and frequently cause oral lesions. The importance of these opportunistic infections has dramatically increased as immunosupressed conditions are more frequent. It is important to consider that mycosis of the mouth can be the primary manifestation of AIDS.