The human vagina is lined by stratified squamous epithelium which has no glands. Major components of the vaginal secretions are transudate through the vaginal walls, desquamating epithelial cells, cervical mucus, fluids from the upper genital tract, and leukocytes. Estrogens and sexual stimulation are examples of factors which increase vaginal fluid. Major organic constituents of the vaginal fluid are proteins, carbohydrates, and fatty acids. Organic acids arise as metabolic byproducts of vaginal bacterial flora, cause the vaginal odor, and show cyclic changes except in oral contraceptive users. Qualitative and quantitative studies in humans as well as animal model studies have confirmed that the vaginal flora is a dynamic and closely interrelated system. Lactobacilli are most prevalent, but many other facultative and anaerobic organisms are present. An inverse relationship exists between the concentration of lactobacilli and other bacteria such as anaerobes. Aerobic bacteria decrease premenstrually whereas anaerobes, in general, remain at constant levels. During menarche, pregnancy, postpartum period, menopause, and postoperative trauma, dramatic changes occur in the microbial flora. The role of an IUD in causing changes in the microbial flora is under great scrutiny at the present time. Overall, disturbances in the vaginal ecosystem may have a potential impact on many diseases, and thus deserve careful studies.