The article is based in part on testimony given at an October 1978 hearing on overcrowded conditions at the New Mexico State Penitentiary. In prisons, crowding is an interacting variable, sometimes causing, sometimes resulting from, and often simply exacerbating the impact of other conditions and practices. Official space standards assume occupancy for about 10 hours per day, but occupancy of 20 to 23 hours is commonly found. The resulting inability of inmates to control privacy and social space can induce stress. Reactions to stress may include aggressiveness, vigilance, guardedness, aloofness, physical disease, and withdrawal. In a crowded prison, both physical and verbal aggression will increase. Single-cell living and dormitories in which 60 prisoners may spend 20 hours per day represent undesirable extremes of privacy. Single cells (six by nine feet) are often used for semipermanent housing of prisoners with identified psychological disturbances. Such prisoners can only deteriorate further in such a setting. Prison crowding both produces exaggerated ways of coping and undermines treatment efforts. Insufficient jobs or programs lower both self-esteem and skills. Organizing into protective groups, aggressive behavior, and attempts to be transferred to single cells are additional responses to crowding. Crowding directly heightens frequency of interpersonal friction and resulting violence. The percentage of prisoners in protective custody indicates the extent to which prisoners live in fear. Although systematic offender classification would be a desirable management tool, overcrowding both prevents its use and exposes its deficiencies. It is concluded that prison overcrowding has multiple negative effects on prisoner adjustment. These effects may occur without crowding but are magnified by it. Current widespread overcrowding should therefore promote reexamination of corrections processes. Footnotes, a reference note, and a reference list are included.