The case for behavior modification and some examples of its application in a variety of correctional settings are presented. The principles, techniques, and strategies utilized in programs designed to induce behavior change are largely determined by the model of causality to which one subscribes. A new approach to human behavior, which is the result of a rapproachement between psychology as a basic and an applied science, represents the first attempt to develop a viable science of human behavior which allows the practitioner to implement experimentally derived and validated principles in an applied setting. The hallmarks of this approach are empiricism and objectivity. Deviant behavior is depicted as acquired in the same manner as normal behavior, and consequently, as amenable to modification through the appropriate use of the laws of learning. Diagnosis is the tool used by more and more professionals to discover the reason for behavioral deviancy. The four functions of diagnosis are: (1) It specifies in what manner an individual differs from those around him; (2) It identifies the causes or origins of the individual's deviance; (3) It supplies information as to what will transpire if no remedial action is undertaken; and (4) It specifies what intervention strategy will be most effective dealing with the deviancy. The guidelines given for use of diagnosis include: (1) a clarification of the problem situation, (2) a motivational analysis, and (3) an analysis of self-control. (CK)