Impact evaluations assess the degree to which changes in outcomes can be attributed to an intervention rather than to other factors. Such attribution requires knowing what outcomes would have looked like in the absence of the intervention. This counterfactual world can be inferred only indirectly through evaluation designs that control for confounding factors. Some have argued that environmental policy is different from other social policy fields, and thus attempting to establish causality through identification of counterfactual outcomes is quixotic. This chapter argues that elucidating causal relationships through counterfactual thinking and experimental or quasi-experimental designs is absolutely critical in environmental policy, and that many opportunities for doing so exist. Without more widespread application of such approaches, little progress will be made on building the evidence base in environmental policy. © Wiley Periodicals, Inc.