Optimising the use of caesarean section (CS) is of global concern. Underuse leads to maternal and perinatal mortality and morbidity. Conversely, overuse of CS has not shown benefits and can create harm. Worldwide, the frequency of CS continues to increase, and interventions to reduce unnecessary CSs have shown little success. Identifying the underlying factors for the continuing increase in CS use could improve the efficacy of interventions. In this Series paper, we describe the factors for CS use that are associated with women, families, health professionals, and health-care organisations and systems, and we examine behavioural, psychosocial, health system, and financial factors. We also outline the type and effects of interventions to reduce CS use that have been investigated. Clinical interventions, such as external cephalic version for breech delivery at term, vaginal breech delivery in appropriately selected women, and vaginal birth after CS, could reduce the frequency of CS use. Approaches such as labour companionship and midwife-led care have been associated with higher proportions of physiological births, safer outcomes, and lower health-care costs relative to control groups without these interventions, and with positive maternal experiences, in high-income countries. Such approaches need to be assessed in middle-income and low-income countries. Educational interventions for women should be complemented with meaningful dialogue with health professionals and effective emotional support for women and families. Investing in the training of health professionals, eliminating financial incentives for CS use, and reducing fear of litigation is fundamental. Safe, private, welcoming, and adequately resourced facilities are needed. At the country level, effective medical leadership is essential to ensure CS is used only when indicated. We conclude that interventions to reduce overuse must be multicomponent and locally tailored, addressing women's and health professionals' concerns, as well as health system and financial factors.