Prison and society represent two different worlds in almost all situations, and the deprivation of liberty presupposes a dramatic change of the existential framework, on the personality of the prisoner, so that, in the article presented "Detainee’s behavior in the penitentiary environment", the emphasis is placed on behavioral changes through the detainee's personality, the characteristics common to the offenders encountered as dominant elements with a certain consistency and frequency, the psychological consequences and the behavioral changes of the detainees that occur due to deprivation of liberty. The article also comes with an analysis of the psychological, analytical, psychosocial, subtraction or self-control theories, social learning regarding the effects of the prison environment on the behaviors of the detainees, mentioning Freud S., T. Hirschi , Walter Reckless, Patterson. It is significant in the process of understanding the personality of the detainee, that from a behavioral point of view, they target the functional and then the causal side. It is necessary to establish what function a certain behavior (fact) performs in the prisoner rather than to inventory the external factors that preceded it (as recommended by the explanation given) and which can sometimes be mistakenly included in the causal sphere.