Caesarean section—the most common surgery in many countries around the world—is a procedure that can save women’s and babies’ lives when complications occur during pregnancy or birth. However, caesarean section use for non-medically indicated reasons is a cause for concern because the procedure is associated with considerable short-term and long-term effects and health-care costs. Caesarean section use has increased over the past 30 years in excess of the 10–15% of births considered optimal, and without significant maternal or perinatal benefits. A three-part Lancet Series on Optimising Caesarean Section Use reviews the global epidemiology and disparities in caesarean section use, as well as the health effects for women and children, and lays out evidence-based interventions and actions to reduce unnecessary caesarean sections.