Uncertainty is the unknown likelihood of a possible outcome. Rice, O’Connor, and Pierantozzi have identified four types of uncertainty: types and amount of resources, technological, market receptivity to the product, and organizational. Each of these uncertainties is present in planning health programs. Ambiguity is doubt about a course of action stemming from awareness that known and unknown factors exist that can decrease the possibility of certainty. In this sense, ambiguity results in uncertainty. Both uncertainty and ambiguity pervade the planning process because it is impossible to know and estimate the effect of all relevant factors—from all possible causes of the health problem, to all possible health effects from program interventions, to all possible acts and intentions of individuals. A rational approach to planning presumes that all relevant factors can be completely accounted for by anticipating the effect of a program, but our experiences as humans tell us otherwise.