The National Aeronautics and Space Administration (NASA) plans to invest billions in the coming years in science and exploration space flight initiatives. The scientific and technical complexities inherent in NASA's mission create great challenges in managing its projects and controlling costs. In the past, NASA has had difficulty meeting cost, schedule, and performance objectives for some of its projects. The need to effectively manage projects will gain even more importance as NASA seeks to manage its wide-ranging portfolio in an increasingly constrained fiscal environment. Per congressional direction, this report provides an independent assessment of selected NASA projects. In conducting this work, GAO compared projects against best practice criteria for system development including attainment of knowledge on technologies and design as well as various aspects of program management. The projects assessed are considered major acquisitions by NASA, each with a life-cycle cost of over $250 million. No recommendations are provided.