The author of this Note speculates about the possibility and desirability of developing a formal calculus for evaluating scenarios. Scenarios are an essential element of military planning. The author argues for the need of a calculus--a rigorous method for evaluation--of the scenarios used in military planning studies. The purpose of the calculus is not to generate scenarios, but to evaluate them within some commonly shared framework or set of rules. A two-dimensional structure for evaluating scenarios is suggested as an illustration of how formal procedures might be employed to diagram a scenario, much as a sentence or electrical circuit is diagramed. A calculus of scenarios, if developed along the lines proposed in the Note, would enable the defender and critic of a scenario to reason together about its omissions, inconsistencies, and uncertainties.