In recent years, governments throughout the world have expressed growing interest in cooperative approaches to environmental protection, including negotiated rulemaking, flexible approaches to enforcement, and voluntary codes and agreements. It is often argued that cooperative approaches are more cost effective, more conducive to innovation, and better able to promote fundamental attitudinal change than traditional “command and control” regulation. However, the overly broad term “cooperative approaches” fails to acknowledge fundamental differences among these novel policies, including distinctions between mandatory and voluntary programs and between those that involve bipartite negotiations between government and business and those that invite participation by a broader range of interests. This article analyzes these cooperative approaches first by offering a framework to distinguish among various cooperative policy instruments. Second, the article critically examines theoretical arguments and empirical evidence concerning one class of cooperative approaches, voluntary challenges and agreements. The most striking finding is how little we know about the effectiveness of voluntary approaches. This is a function not only of the quite recent experience with these approaches, but also of more fundamental inattention to program evaluation and obstacles to evaluation inherent in voluntary programs. The article concludes with a call for a more rigorous program of research to examine the effectiveness of the new policy instruments and to compare them with traditional regulation and market-based incentives.