The Organisation for Economic Co-operation and Development (OECD) has been successful in framing and transferring the knowledge of regulatory reform. As an ideational authority, its activities have promoted the adoption of regulatory impact assessment (IA) among developed countries. However, administrative and evaluation practices of IA are based on national administrative traditions and country-to-country interaction. This chapter relies on the concept of transnational policy innovation that allows the author to address the questions of what has been diffused and when the spread has occurred. By putting emphasis on transnational networks, this chapter also provides a useful discussion about the OECD practices for diffusing policy innovations. The conclusion suggests that the OECD should promote Member States to exchange learning experience on how to fit transnationally transferred innovations to their administrative systems.