Late preterm infants, defined as infants delivered from 34 through 36 completed weeks’ gestation, are less mature developmentally and physiologically than term infants (37 to 41 completed weeks’ gestation). Such immaturity places infants born late preterm at higher risk of morbidity and mortality than term infants during infancy and potentially throughout life. The purpose of this article is to review the epidemiology of infants born late preterm by describing the frequency of late preterm births; trends over time; and differences according to maternal age, race, and ethnicity. In addition, the article describes limitations associated with accurate estimation of gestational age and explains how these limitations may affect understanding of the true frequency of late preterm birth. Finally, the evidence of neonatal, postneonatal, and childhood morbidity risk associated with being born late preterm is compared with that of infants born at term.