The Defense Department's Joint Strike Fighter (JSF) is one of three aircraft programs at the center of current debate over tactical aviation, the others being the Air Force F/A-22 fighter and the Navy F/A-18E/F fighter/attack plane. In November 1996, the Defense Department selected two major aerospace companies, Boeing and Lockheed Martin, to demonstrate competing designs for the JSF, a joint-service and multi-role fighter/attack plane. On October 26, 2001, the Lockheed Martin team was selected to develop further and to produce a family of conventional take-off and landing (CTOL), carrier-capable (CV), and short take-off vertical landing (STOVL) aircraft for the U.S. Air Force, Navy, and Marine Corps and the U.K. Royal Navy as well as other allied services. Originally designated the Joint Advanced Strike Technology (JAST) program, the JSF program is a major issue in Congress because of concerns about its cost and budgetary impact, effects on the defense industrial base, and implications for U.S. national security in the early 21st century. The JAST/JSF program evolved in response to the high cost of tactical aviation, the need to deploy fewer types of aircraft to reduce acquisition and operating costs, and current projections of future threat scenarios and enemy capabilities. The program's rationale and primary emphasis is joint-service development of a nextgeneration multi-role aircraft that can be produced in affordable variants to meet different operational requirements. Developing an affordable tri-service family of CTOL and STOVL aircraft with different combat missions poses major technological challenges. Moreover, if the JSF is to have joint-service support, the program must yield affordable aircraft that can meet such divergent needs as those of the U.S. Air Force for a successor to its low-cost F-16 and A-10 fighter/attack planes, those of the U.S. Marine Corps and the U.K. Royal Navy for a successor to their Harrier STOVL aircraft, and the U.S. Navy's need for a successor to its carrier-based F-14 fighters and A-6 attack planes and a complement to its F/A-18E/F fighter/attack planes. This report discusses the background, status, and current issues of the JSF program. Continuing developments and related congressional actions will be reported in CRS Issue Brief IB92115, Tactical Aircraft Modernization: Issues for Congress, which also discusses the Air Force F/A-22, the Navy F/A-18EF, and the Marine Corps V-22. These aircraft and the Air Force's B-2 strategic bomber and C17 cargo/transport plane are the most expensive U.S. military aircraft programs. (See CRS Report 95-409 F, Long-Range Bomber Facts: Background Information, and CRS Report RL30685, C-17 Cargo Aircraft Program updated periodically.) The JSF program is also addressed in CRS Report RS21488, Navy-Marine Corps Tactical Air Integration Plan: Background and Issues for Congress, and CRS Report RL31360, Joint Strike Fighter (JSF): Potential National Security Questions Pertaining to a Single Production Line.