In response to a congressional request, GAO evaluated the Federal Aviation Administration's (FAA) National Airspace System (NAS) Plan to determine: (1) the additional resources that modernization would require before realizing its benefits; (2) the causes and effects of development delays that the most costly and complex NAS Plan projects have experienced; and (3) FAA weaknesses in managing the plan. GAO found that: (1) although FAA awarded contracts to develop or produce 80 of the 92 projects, including 8 of the major systems needed to upgrade the air traffic control (ATC) system, the projects were an average of 3 years behind schedule; (2) because new projects and changes to existing projects expanded the extent of modernization, FAA would need at least $25 billion in appropriations by the year 2000; (3) FAA lacked experience in developing and integrating large-scale systems and put several of its major systems into full production without adequate testing and evaluation; (4) FAA underestimated the size and complexity of the development effort, which led to additional performance requirements and software design difficulties; (5) FAA overstated some projects' benefits, which made trade-offs difficult among projects whose benefits had high passenger-time-savings components; (6) FAA runs the risk that the planned testing of its major systems will not be objective due to program managers' competing goals of achieving a timely, working system within budget; (7) FAA implemented a long-range planning policy to ensure that it would effectively integrate the separate plans for the interrelated NAS components; and (8) the NAS Plan needed revisions to include all the projects for modernization, correctly estimate many project benefits, and better coordinate the NAS Plan with other FAA plans for building airports, making airspace changes, and managing human resources.