The article compares the interplay between soft law institutions and those based on hard law in international efforts to protect the North Sea, reduce trans-boundary air pollution, and discipline fisheries subsidies. Our cases confirm that ambitious norms are more easily achieved in soft law institutions than in legally binding ones, but not primarily because they bypass domestic ratification or fail to raise concerns for compliance costs. More important is the greater flexibility offered by soft law instruments with respect to participation and sectoral emphasis. Second, ambitious soft law regimes put political pressure on laggards in negotiations over binding rules, but this effect is contingent on factors such as political saliency and reasonably consensual risk and option assessment. Third, hard-law instruments are subject to more thorough negotiation and preparation which, unless substantive targets have been watered down, makes behavioral change and problem solving more likely. Finally, although most of the evidence presented here confirms the implementation edge conventionally ascribed to hard law institutions, the structures for intrusive verification and review that provide part of the explanation can also be created within soft law institutions. [ABSTRACT FROM AUTHOR]