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With the growing number of rigorous impact evaluations worldwide, the question of how best to apply this evidence to policymaking processes has arguably become the main challenge for evidence-based policymaking. How can policymakers predict whether a policy will have the same impact in their context as it did elsewhere, and how should this influence the design and implementation of policy? This paper introduces a simple and flexible framework to address these questions of external validity and policy adaptation. I show that failures of external validity arise from an interaction between a policy’s theory of change and a dimension of the context in which it is being implemented, and develop a method of “mechanism mapping” that maps a policy’s theory of change against salient contextual assumptions to identify external validity problems and suggest appropriate policy adaptations. In deciding whether and how to adapt a policy in a new context, I show there is a fundamental informational trade-o↵ between the strength and relevance of evidence on the policy from other contexts and the policymaker’s knowledge of the local context. This trade-o↵ can guide policymakers’ judgments about whether policies should be copied exactly from elsewhere, adapted, or invented anew.