The social science concept of crowding became prominent in the field of criminal justice as the prison population rose in the late 1970's. By the time a penal crisis was declared in the 1980's, the study of crowding had divided into subfields that focused on the management and effects of crowding. By the early 1990's, these subfields came back together. Current interest in crowding is due to several reasons, such as humanitarian concerns, interest in the constitutionality of prison conditions, the fact that crowding has not been established as an independent legal concept, costs and benefits of incarceration policies, and the empirical examination of crowding effects. One of the most important criticisms of the usefulness of crowding research relates to the validity of the construct itself. As crowding research has progressed, conceptual problems in defining and measuring prison populations have emerged. Crowding can be considered at the individual level or at the aggregate facility level. Operationalizing the concept, however, reveals construct validity problems at both levels. Crowding comparisons across facilities are complicated by variations related to facility layouts, space availability, schedules, supervision styles, and other factors. Dependent variables most commonly considered in the context of crowding include violence and prisoner health, and a range of intervening variables must be considered in efforts to describe the effects of crowding. The authors conclude that crowding research has little significance for public policy due to methodological problems, the political context of public policy, and the failure of crowding research to clearly demonstrate the negative effects of crowding. 22 references