Habitability is a general term which connotes a level of environmental acceptability. The requirements for conditions to be "habitable" change dramatically with circumstances. For brief periods, almost any arrangement that does not interfere with the health of the individuals or the performance of their jobs would be acceptable. Over the long term, conditions must support not only individuals' physical, but also their psychological health.
Historically, habitability research has focused on the impact of environmental factors. It was assumed, for instance, that the presence of a noise stimulus or the confinement of large numbers of individuals in a small space was sufficient to explain performance variations. In a second phase of inquiry, attention turned towards personal or psychological factors as also being important in determining how individuals respond to less than ideal conditions (see review by Cohen, Glass, and Phillips, 1979). The most recent developments in this research area have shifted the focus away from the components of habitability (i.e., the individual and the environment) to the relationship between them (i.e., the person environment fit). According to this model, people are motivated towards certain goals which, if achieved, result in a sense of well being. The ability to reach these goals depends both on characteristics of the individual and on the supplies of the environment. A mismatch between an individual's values and abilities and the offerings of the environment results in strain, and eventually in physical and psychological illness. Evidence  shows the person environment fit model can have predictive value over and above that of the components (Harrison, 1978; French, Rodgers, and Cobb,1974).
In this chapter we will consider some of the habitability issues of extended spaceflight. Because of the extreme nature of the space environment, the emphasis here must necessarily be environmental. However, as the chapter progresses, we will attempt to highlight both the personal factors that affect adaptation to space and the interaction between personal and environmental factors.