As a result of exposure to the external environment, the human vagina is home to many diverse bacterial species. The composition of the latter varies according to the life stage of the individual female due to differences in hormone production and lifestyle. Advances in bacterial detection using culture-independent methods have greatly altered our knowledge of the composition of the vaginal microbiome especially in nonpregnant reproductive age women. The vaginae of approximately 80% of healthy women are dominated by one of four species of Lactobacillus: L. crispatus, L. iners, L. gasseri, and L. jensenii. The remaining 20% are dominated by a diverse group of anaerobic and facultative bacteria. A predominant characteristic of all bacterial populations in healthy women is the production of lactic acid. The resulting acidification of the vagina retards or prevents the growth of other potentially pathogenic bacteria. Lactic acid also has specific effects on vaginal epithelial cells and immune cells to further protect against pathogen establishment. The interactions between bacterial species as well as their ability to produce various metabolites also influences the composition of the host vaginal immune and biochemical milieu and influences susceptibility to various lower genital tract infections as well as the capability of bacteria to transcend the cervical barrier and invade the upper genital tract. In pregnant women migration of bacteria from the vagina to the uterus is a major cause of preterm birth. A more complete understanding of the composition of the vaginal microbiome and its influence on host processes will lead to development of new protocols utilizing novel reagents to improve the health of pregnant as well as nonpregnant women.