The last decade has witnessed a demand for evidence-based programs in virtually every field of social research. Presumably evidence will help inform officials as to which programs are likely to yield successful outcomes as well as help practitioners operate programs with best practices. But program effectiveness is only half the answer. The other half is affordability. Policymakers make budget-constrained decisions. A decision to implement a program in one area means cuts in programs in some other area. Evaluations that report only effectiveness findings cannot contribute much toward social program decisionmaking. Evaluators must start to provide information on both costs and effectiveness or costs and benefits. This paper presents some of the challenges of incorporating cost benefit analysis into traditional criminal justice program evaluations. It presents illustrations of the conceptual and measurement issues to be faced evaluating programs in such areas as private security, juvenile delinquency, police interventions, and correctional rehabilitation when researchers attempt to add cost analysis to program inputs and try to convert outcomes into monetary units. It raises issues regarding availability, program externalities, hidden resources, and inadequacies of outcome measures. It concludes with some general guidance for evaluators on conducting such analyses and a checklist of questions to consider when deciding between cost effectiveness analysis and cost benefit analysis. References (Published Abstract)