This analysis uses data from Selected Acquisition Reports to determine the causes of cost growth in 35 mature major defense acquisition programs. Four major sources of growth are identified: (1) errors in estimation and scheduling, (2) decisions by the government, (3) financial matters, and (4) miscellaneous. The analysis shows that more than two-thirds of cost growth (measured as simple averages) is caused by decisions, most of which involve quantity changes, requirements growth, and schedule changes.