Since the earliest days of spaceflight, substantial concern has been expressed regarding the physical needs of astronauts, including any biological damage that might result from exposure to radiation or from reduction in gravitational forces. In contrast, relatively little concern has been directed towards people's psychological and social adjustment to space. At one time this difference in emphasis was justified. The Mercury, Gemini, and Apollo flights were measured in hours and days and it could be reasonably assumed that astronauts would be able to withstand certain deprivations for these brief periods. The longer flights of Skylab presented a different picture. Early in the development of Skylab, it was recognized that steps would have to be taken to accommodate a wider variety of human needs. However, the needs that were addressed remained narrowly defined and centered primarily on habitability considerations. We are now at the point in the development of spaceflight where the range of psychological and social requirements of the human participant must be given full consideration. There is hope that NASA soon will move ahead with its long-awaited space station. As this is written, information from the Russian space program suggests that the Soviet Union will soon launch a space vehicle capable of carrying a crew of twelve, possibly as a prelude to a manned Mars mission. Whatever the specific projects or time frames, it seems clear that tomorrow's manned spaceflights will involve large numbers of people living and working together under close confines and in "unnatural" environments for long periods of time. Adjustment to such conditions has important implications for mental health, for social organization, and ultimately for mission success. Because tomorrow's astronauts are likely to expect, and even demand, greater autonomy in living and working arrangements, the planners' perspective must extend beyond concern for effective functioning within the space community and encompass the relationship between the space community and the home planet. In this book we attempt to identify and assess, in a serious and systematic fashion, the psychological and social problems that may be associated with future space missions, and to explore some possible solutions. This task involves establishing both a structure in which relevant issues can be considered and a level of analysis that can contribute to a scientifically based understanding of human adaptation to space.