Iraqi Scud missile attacks during the Persian Gulf War dramatized U.S. vulnerability to theater ballistic missiles. In this report, the authors describe a methodology for allocating resources among various facets of missile defense. Their model takes into account the number of attacking objects, the requirement that no missiles get through, the number of layers of defense, the probability of kill by the interceptors in each layer, other operational parameters, and cost considerations. In addition, they examine the effects of critical uncertainties. The model indicates the benefits of architectures that rely on multiple layers of defense, the high costs of these defenses, and the high leverage in engaging attacking objects before launch or early in their flight.