Inspired by the success of evidence-based medicine, environmental scholars and practitioners have grown enthusiastic about applying a similar evidence-based approach to solve some of the world's most pressing environmental problems. An important component of the evidence-based movement is the empirical evaluation of program and policy impacts. Impact evaluations draw heavily from recent advances in the empirical study of causal relationships—the effect of one thing on another. This review highlights the key components of these advances and characterizes the way in which they contribute to better evaluations of the environmental and social impacts of environmental programs. The review emphasizes that a solid understanding of these advances is required before environmental scholars and practitioners can begin to collect the relevant data, analyze them within credible research designs, and generate reliable evidence about the effectiveness of the myriad proposed solutions to the world's environmental and social problems.