Preterm birth complicates 8-10% of all pregnancies in the United States and is the leading cause of infant morbidity and mortality. Neonatal morbidity and mortality is concentrated among very low-birthweight and extremely premature infants, particularly those delivered prior to 30 weeks' gestational age. In addition to the contribution of preterm birth to neonatal morbidity and mortality, the economic costs associated with this pregnancy complication are staggering. Efforts to reduce the preterm birth rate have been largely focused on prevention and early intervention with treatment for preterm labor. Mixed results regarding the success of prematurity prevention programs have been reported, and controversy continues to surround the efficacy of tocolytic therapy in the treatment of preterm labor. Although neonatal survival for infants born at early gestational ages has steadily improved in recent years, survival of infants delivered prior to 24 weeks' gestation remains very poor. Additionally, despite this decline in neonatal mortality, the United States still lags behind most industrialized nations in infant mortality, and no change in the rate of low birthweight has occurred in recent decades. Multiple lines of evidence support a role for infection as an etiologic factor in preterm labor. Although this association has been well known for many years, a wealth of new data is emerging, linking subclinical genital tract infection with spontaneous preterm birth, particularly among pregnancies that result in birth prior to 30 weeks' gestational age as a result of spontaneous preterm labor or preterm, premature rupture of membranes. Conversely, preterm birth that occurs closer to term is less likely to be associated with genital tract infection. Improved understanding of the link between genital tract infection and preterm birth now provides an exciting potential for the development of sensitive new markers to identify women at risk and effective interventions to prevent preterm birth. A review and comment on this growing literature is provided.