This article is a part of a series that focuses on the current state of evidence and practice related to preterm birth prevention. We provide an overview of current knowledge (and limitations) on the global epidemiology of preterm birth, particularly around how preterm birth is defined, measured, and classified, and what is known regarding its risk factors, causes, and outcomes. Despite the reported associations between preterm birth and a wide range of socio-demographic, medical, obstetric, fetal, and environmental factors, approximately two-thirds of preterm births occur without an evident risk factor. Efforts to standardize definitions and compare preterm birth rates internationally have yielded important insights into the epidemiology of preterm birth and how it could be prevented.