This chapter examines the diffusion and transfer of impact assessment (IA) in developing countries. ‘Diffusion’ is used to refer to the spread of IA from Organisation for Economic Co-operation and Development (OECD) countries to developing countries. This spread of IA to developing and transition countries is first described. This is followed by a discussion of the ‘transfer’ of IA to lower- and middle-income economies. Here, transfer is used to refer to the successful adoption and implementation of IA. This allows for a discussion the problems of inappropriate policy transfer in the context of diffusion of the ‘best practice’ OECD IA model. The key factors that will affect the successful transfer and sustainability of the IA approach to developing countries is then discussed. The chapter ends with a brief summary and conclusion.