Preterm delivery accounts for a substantial percentage of perinatal mortality and morbidity. A large body of evidence suggests that perinatal infections may play an etiologic role in preterm rupture of membranes and preterm labor. The interaction between a patient's vaginal flora and her local defense mechanisms (antibacterial activity of amniotic fluid and cervical mucus) may help determine who delivers prematurely. The evidence implicating infection as a cause of labor is reviewed, and the organisms which may be responsible are identified. Potential avenues of investigation and intervention are outlined which would delineate the role of infections in preterm delivery and lead to a means of preventing some preterm deliveries caused by infection.