This research is part of the Management and Outcomes of Childbirth Patient Outcomes Research Team (PORT), which is supported by the Agency for Health Care Policy and Research. This report summarizes the literature on variations in the use of cesarean section for childbirth, including variations over time, geographic variations, and clinical and nonclinical correlates of variations. The literature describes dramatic increases in aggregate C-section rates over time, and dramatic differences among countries, regions, states, and smaller areas. The changes over time can be largely attributed to changes in the four major diagnoses that lead to most C-sections: more frequent diagnoses of prior C-section, dystocia, fetal distress, and higher C-section rates for breech diagnoses. But these attributions still fail to explain why C-section rates have increased so dramatically in the last two decades.