This study examines the role of prototypes in the contemporary environment of weapon system acquisition. The research draws on case studies of four systems (two Air Force airplanes and two Army helicopters) that were developed in the early 1970s and that used prototypes in various ways. These were compared with a broad range of acquisition programs that used other acquisition strategies. The objective of the study is to sharpen the understanding of advantages and disadvantages of prototyping and conditions under which its use may be advantageous. Section II presents an outline of the different kinds of prototypes, and the various objectives that might be sought in a prototype phase. The section concludes with a description of the analysis procedure, a summary of the four systems examined, and the source of data on nonprototype programs used for comparison. Section III summarizes the results of the research, and Sec. IV contains the conclusions. Four appendixes are attached, each describing one of the case studies.