During the 1990s a new regulatory pattern in domestic environmental policymaking has been emerging which is characterized by an increasing use of cooperative, informational and marked-based instruments. This pattern is to an important extent a result of international policy diffusion the cross-national spread of policy innovations driven by information flows rather than hierarchical or collective decision-making within international institutions. Based on four case studies, the paper demonstrates empirically how horizontal diffusion processes accompanied by information and recommendations from international organizations have led to the adoption of new regulatory instruments in an increasing number of countries and how these individual national adoptions add up to an emerging regulatory structure at the international level. At the macro-level, the case studies explore how diffusion interacts with the other two major international mechanisms of domestic policy change: legal harmonization and coercive imposition. Especially within the European Union a typical pattern of horizontal diffusion between individual member states, followed by vertical diffusion from the national to the EU-level and finally leading to an EU-wide legal harmonization through EC-directives can be identified. At the micro-level, the paper investigates which factors promote or obstruct the diffusion of new environmental policy instruments. While the endorsement of regulatory instruments by international organizations or transnational advocacy networks often facilitates their diffusion, the instruments' characteristics determine the extent and speed by which regulatory instruments spread across countries. As regards policymakers' motivation for voluntarily adopting regulatory instruments, the paper argues that it cannot be exclusively explained by rational attempts to improve policy effectiveness. In addition, policy adoption is often motivated by concerns of legitimacy and perceived pressure to conform with international norms.