This chapter introduces several debates surrounding the effectiveness of global environmental governance. These debates are closely linked to the choice of policy instruments states make within international regimes. These public policy instruments include regulations, administrative standards, scientific indicators, financial targets, and accounting practices, among others. Whereas international institutions frame the general norms, principles, and rules for tackling environmental problems, instruments provide the toolbox of policy mechanisms that actors in global environmental politics use to implement those norms, principles, and rules. In some cases, the choice of instruments is made at the international level and applied in exactly the same way by a group of states. In other cases, the choice of policy instruments is left to the discretion of states, who can then choose among different alternatives to fulfil their international commitments. The chapter then explains the modalities, diffusion, and political effects of these policy instruments. Although the concept of policy instruments may appear technical and neutral, it shows how instruments can actually shape, modify, and even undermine global environmental politics.