Applying risk analysis to strategies against terrorism requires an innovative combination of approaches. Assessing the risk of terrorism (which is dynamic and adaptable) is fundamentally different from assessing the risk of accidents or acts of nature. In particular, terrorists can adopt different strategies in response to our defenses. Game theory, an economic tool for mathematically modeling optimal strategies, provides a way of accounting for this. Counterterrorism can benefit from the combination of risk analysis and game theory. While game theory quickly becomes mathematically complex and may not be applicable to problems of realistic scale and complexity, the results of such analyses can yield useful qualitative insights into the nature of optimal defensive investments.