Rates of preterm birth range from 5% to 13% of deliveries in developed countries. About two-thirds of preterm deliveries are due to spontaneous onset of preterm labour or preterm premature rupture of membranes. Approximately one-third follow induction of labour or caesarean section performed for maternal or fetal indications such as preeclampsia, haemorrhage, non-reassuring fetal heart rate or intrauterine growth restriction. Thus, pathologists are frequently called on to evaluate preterm placentas, to determine the cause of the spontaneous preterm birth and/or correlate placental findings with the clinical history. This review provides pathologists with an overview of the recent clinical research in the pathogenesis of preterm birth and relates these to the correlative placental pathologies of the major causes of spontaneous preterm birth. A brief summary of the placental gross and histopathological findings in indicated preterm birth is also included.