In considering the relative efficiency of punishment severity and punishment probability in reducing crime, information is needed on the degree of diminishing returns in the relation between punishment severity and crime, the expected value of crime-committing costs, the role of crime benefits, the monetary and nonmonetary costs of punishment and its probability, and alternative punishment types and methods of increasing punishment probability. Factors that must be considered in predicting the effects of changes in punishment severity and probability are the interaction between severity and probability, the feedback effect of crime on severity and probability, and the role of crime benefits. The aforementioned factors can be integrated into a set of equilibrium and optimum models. The three equilibrium models pertain to the reciprocal causation between punishment severity and crime, punishment probability and crime, and crime benefits and crime. The two optimizing models focus on optimum crime levels for criminals and for society. The article concludes with a list of information items required to make cost-effective decisions on crime reduction policy. 5 notes and 23 references.