The measure of the effectiveness of health promotion and disease prevention activities is the impact of prevention policies, programs, and practices on public health and clinical medicine. Assessing prevention effectiveness involves continuing quantitative analysis of health outcomes resulting from prevention practices. Additionally, assessment involves evaluation of disease- and injury-prevention activities, including their medical, legal, ethical, and economic impacts. Although assessing the effectiveness of prevention activities involves measuring efficacy, safety, and cost, the primary criterion is to improve health at a reasonable cost, not merely to contain costs. Policy makers can use the results of assessments to set priorities in public health. The authors use case studies to illustrate various approaches to evaluating prevention programs, including programs for preventing measles, breast cancer, and diabetic retinopathy. Rigorous evaluation of the effectiveness of prevention activities is essential to the wide acceptance of preventive interventions and the willingness to pay for them.