Crowding can be measured objectively in terms of floor space per prisoner, prisoners per living unit, and institutional population relative to stated capacity. Whether an inmate perceives conditions as crowded depends on objective conditions and on relative differences in crowding within a prison's housing accommodation. Research on prison crowding has not convincingly demonstrated many adverse effects of crowding. Most researchers agree that (1) prisoners housed in large open-bay dormitories are more likely to visit clinics and have high blood pressure than are those in other arrangements, (2) prisons containing dormitories have somewhat higher assault rates than do other prisons, and (3) those housing significantly more prisoners than a design capacity based on 60 square feet per inmate are likely to have high assault rates. The relationship of crowding to the distribution and availability of prison resources has not been investigated. Crowding may act as an intensifier of stressful conditions that have been precipitated by other causes. Under extreme conditions, crowding can itself produce stress reactions. The limited number of research findings that can be asserted with confidence is the product of inherent difficulties confronting efforts to conduct well controlled studies in prisons. Included are 8 tables and 70 references. (Author abstract modified)