The United States Air Force, Navy, and Marine Corps have long maintained tactical fighter forces that provide capabilities for air-to-air combat and air-to-ground attack. The three services are in the process of replacing the bulk of today's fighter aircraft, most of which were purchased in the 1980s, with new F/A-18E/F, F-22, and F-35 (Joint Strike Fighter) aircraft. Although current procurement plans call for the purchase of about 2,500 aircraft over the next 25 years, the services are projecting that those purchases will not keep pace with the need to retire today's aircraft as they reach the limit of their service life. The study also compares the advantages, disadvantages, and costs of seven alternative approaches that DoD might adopt to modernize its fighter forces -- three that satisfy today's inventory requirements, two that maintain aggregate weapons capacity with fewer aircraft, and two that replace portions of the fighter force with longer-range aircraft. In keeping with CBO's mandate to provide objective, impartial analysis, this study makes no recommendations.