Environmental factors which may increase violent behavior are categorized as spatial intrusions, monotony, and external control. The stress caused by these factors is likely to increase violent reaction especially in those individuals already predisposed to it. Studies have found that all persons display territoriality, which varies according to situation, relationship to other persons, ethnic background, and other variables. The prison setting intensifies normal responses to invasion of space and triggers violent reaction to it. A clearer understanding of the spatial needs of different prisoners could reduce the tendency toward aggressiveness. Monotony in an environment lacking sources of stimulation leads prisoners to create their own excitement, which is frequently undesirable. Simple changes in architectural and interior design, and variety in food and recreational opportunities are presented as possible solutions. The third factor discussed, lack of control over one's environment, is also largely an integral part of the prison conditions and one which many find threatening. Violent reactions often occur when the simplest aspects of daily life are beyond one's control. Although environmental changes alone would not eliminate violence, appropriate changes would reduce it. In addition, unavoidable violence might then become more predictable and subject to control. Over 20 references are provided.