Reproductive success is critical for survival of the species. The timely onset of labor and delivery is an important determinant of perinatal outcome. Both preterm birth (defined as delivery before 37 weeks' gestation) and post-term pregnancy (defined as pregnancy continuing beyond 42 weeks) are associated with a significant increase in perinatal morbidity and mortality. The factors responsible for the timing of labor in the human are complex and, as yet, are not completely understood. This article reviews the current understanding of the parturition cascade responsible for the spontaneous onset of labor at term and discusses preterm labor and post-term pregnancy.