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: errors in estimation and scheduling, decisions by the government, financial matters, and 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.Cost growth in major weapon-systems programs results from errors in estimation and scheduling, government decisions, financial matters, and miscellaneous sources, with decisions involving changes in requirements, quantities, and production schedules the dominant cause.