Each year in the United States approximately 440,000 babies are born premature. These infants are at greater risk of death, and are more likely to suffer lifelong medical complications than full-term infants. Clinicians and researchers have made vast improvements in treating preterm birth; however, little success has been attained in understanding and preventing preterm birth. Understanding the complexity of interactions underlying preterm birth will be needed if further gains in outcomes are expected. The Institute of Medicine's Roundtable on Environmental Health Sciences, Research, and Medicine sponsored a workshop to understand the biological mechanism of normal labor and delivery, and how environmental influences, as broadly defined, can interact with the processes of normal pregnancy to result in preterm birth. This report is a summary of the main themes presented by the speakers and participants.