An overview is presented of the kinds of anaerobic bacteria that inhabit the surfaces of the human body. The anaerobic floras of the skin, oral cavity, alimentary tract, and genitourinary tract are described. The activities of these organisms that impact on the human host and their interactions are discussed. Emphasis is placed on the protective roles of the floras of the various bodily surfaces in preventing infections. Identified mechanisms of protection on the skin and in the colon and the vagina are explained.