Principal Investigator-Led (PI-led) missions are an important element of NASA’s spacescience enterprise. While several NRC studies have considered aspects of PI-ledmissions in the course of other studies for NASA, issues facing the PI-led missions ingeneral have not been subject to much analysis in those studies. Nevertheless, theseissues are raising increasingly important questions for NASA, and it requested the NRCto explore them as they currently affect PI-led missions. Among the issues NASA askedto have examined were those concerning cost and scheduling, the selection process, relationships among PI-led team members, and opportunities for knowledge transfer tonew PIs. This report provides a discussion of the evolution and current status of the PIledmission concept, the ways in which certain practices have affected its performance, and the steps that can carry it successfully into the future. The study was done incollaboration with the National Academy of Public Administration.