This report is one of a series from a RAND Project AIR FORCE project, "The Cost of Future Military Aircraft: Historical Cost Estimating Relationships and Cost Reduction Initiatives." The report complements another document from this project, "Impossible Certainty: Cost Risk Analysis for Air Force Systems," and includes a literature review of cost growth studies and a more extensive analysis of the historical cost growth in acquisition programs than appears in the companion report. Overall, most of the studies reviewed reported that actual costs were greater than estimates of baseline costs. The most common metric used to measure cost growth is the cost growth factor (CGF), which is defined as the ratio of the actual cost to the estimated costs. A CGF of less than 1.0 indicates that the estimate was higher than the actual cost -- an underrun. When the CGF exceeds 1.0, the actual costs were higher than the estimate -- an overrun. Studies of weapon system cost growth have mainly relied on data from Selected Acquisition Reports (SARs). These reports are prepared annually by all major defense acquisition program (MDAP) offices within the military services to provide the U.S. Congress with cost, schedule, and performance status. The comparison baseline (estimate) typically corresponds to a major acquisition decision milestone. Prior studies have reported Milestone (MS) II CGFs for development costs ranging from 1.16 to 2.26; estimates of procurement CGFs ranging from 1.16 to 1.65; and total program CGFs ranging from 1.20 to 1.54. Regarding the differences among cost growth due to service, weapon, and time period, prior studies tended to find the following: Army weapon systems had higher cost growth than did weapon systems for the Air Force or Navy; cost growth differs by equipment type; and cost growth has declined from the 1960s and 1970s, after it was recognized as an important problem.