Enclosure of individuals within a limited space may give rise to problems devolving from the associated restriction or constriction, as modified by any accompanying elements of social isolation and perceptual deprivation. In this paper, the nature of the resulting problems is outlined on the basis of analysis of some 60 studies of operational and experimental human confinement in which the associated elements of isolation and perceptual deprivation were considered minimal, along with consideration of other related work. It is pointed out that in the operational situations so far encountered, and in experimental studies in which highly trained and well-motivated subjects undergo realistic space-mission simulation, undesirable psychological effects have been negligible. Physiological effects are similar to those found in actual and simulated weightlessness, and are dependent largely upon the extent of immobility and the duration of the confinement. The response to confinement, including those undesirable psychological effects which are primarily associated with the extent of social isolation and perceptual deprivation, is modified by factors within the individual, such as motivation, personal qualities, training and experience, by the quality of leadership and group compatibility, by physical fitness of the crew and the use of exercise regimens, by the suitability and habitability of the immediate environment, the suitability of work and rest schedules, the meaningfulness of the activities and tasks in which the subjects or crew are engaged, by the existence of facilities for recreation and the creative use of leisure, and by informed knowledge of the status of the mission. The requirements for free internal volume of space vehicles for long-duration (400-day) missions cannot be stated with accuracy at this time, but from examination of confinement studies and crew-activity analyses, it is suggested that the minimum free volume per man in a multi-man crew would be about 200–250 cubic feet; the acceptable would be about 350–400 cubic feet, and the optimal about 600–700 cubic feet.