Epidemiology can inform decision-making by providing projections of the impact of potential interventions on the health of the population. Computer simulations of health and disease are very useful here, given their ability to superimpose the epidemiologic processes upon the underlying population dynamics, dealing with far more variables that the human mind can manage, and considering both beneficial and adverse effects and subtle phenomena like the compression of morbidity. Thereby, they can provide answers to “What if” questions like “What would be the effect on the overall health of the population if we reduced smoking by 20%?”, Better yet, they can often compare the probable health impact of different interventions, and provide the basis for the economic analyses that will help policy-makers choose among them. Epidemiology can also assist the process of consensus development for selection of a particular policy, using priority-setting techniques borrowed from the social sciences.