Of the 196 described species of Candida (including Torulopsis) only ten, are isolated with any frequency from medical specimens. Starting with the most pathogenic, these comprise: Candida albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. pseudotropicalis, C. guilliermondii, C. viswanathii, and C. lusitaniae. Candida represents the most common genus occurring in the oral cavity, and C. albicans the most often recovered species. The chapter provides an historical review of the emergence of candidosis, and discusses common predisposing factors and conditions which alter host defenses and predispose to Candida infection. It includes a review of the mechanisms by which these factors may predispose to candidosis. From the early seventies onwards research was directed towards understanding the mechanisms by which various predisposing factors enhance the pathogenicity of Candida and how to control its infection. Infection with Candida follows alterations of the host defense systems, or environmental changes at particular sites in the human body.