An unintended consequence of the rising cesarean section rate is abnormal placentation in subsequent pregnancies, leading to the clinical complications of placenta accreta and cesarean scar pregnancies. Both of these clinical entities are associated with high rates of maternal morbidity and mortality. This article reviews the potential mechanisms by which uterine scarring may lead to abnormal trophoblast invasion, the association of cesarean section with placenta accreta and scar pregnancies, current management, and suggestions for future research to reduce the incidence of these potentially devastating complications of pregnancy.