In this chapter we review the historical and current practices guiding the psychological selection of astronauts as a means of informing the broader challenge of selecting personnel for work in extreme environments in general. We outline limitations preventing the use of more traditional approaches to developing and validating effective selection practices in the spaceflight environment, then discuss the state of supporting evidence and reflect on how to adapt these practices given new long-duration spaceflight mission concepts and the increasingly international composition of spaceflight crews. We conclude with ideas for improving psychological selection processes for astronauts as space agencies work to successfully execute exciting, increasingly complex and ambitious space missions in the future.