This article examines the effectiveness of environmental policies. Do environmental policies work? This may seem to be a simple, straightforward question. To anyone involved in the environmental policy process, it is certainly an important one--yet for many environmental policies, it lacks a solid answer. Decisionmakers often lack carefully collected evidence about what policies have accomplished in the past---and in this sense they are poorly informed about what new policies might accomplish in the future. Getting systematic answers to the question of whether environmental policies work is vital. Real resources are expended on environmental regulatory programs, and at a minimum one should expect that these programs then lead to improvements in environmental conditions. While intuitions and anecdotes may provide some reason for suspecting that a given policy has made or will make a difference, the only way to be confident of such suspicions is to evaluate a policy's impact in practice. Program evaluation research provides the means to determine what has and has not worked and thereby to decide whether to retain existing policies or adopt new or modified ones. Academics, policymakers, activists, and business leaders do generally recognize the need for careful evaluation of existing environmental policies. Indeed, some important research has been undertaken, particularly studies of the effects of long-standing regulations such as the Clean Air Act and the Clean Water Act. Yet program evaluation research has been remarkably scarce relative to the overall volume of environmental policy decisions made at the state and federal level and to the amount of evaluation research found in other fields, such as medicine, education, or transportation safety. A renewed and greatly expanded commitment to program evaluation of environmental policy would help move environmental decisionmaking closer to an evidence-based practice. INSET: ILLUSTRATIVE EVALUATION OF AN ENVIRONMENTAL POLICY.