This chapter emphasizes the importance of mental and social well-being in the success of both short and long space missions. What are the psychological and sociological issues, which must be addressed, especially for international missions? This section reviews the factors that may have a critical impact on the success or failure of a space mission, in terms of interactions of the crewmember with his habitat, with the space environment, and with the other crewmembers (Fig. 7.1).