The contribution made by innovation and new technologies to economic growth and welfare is largely determined by the rate and manner by which innovations diffuse throughout the relevant population, but this topic has been a somewhat neglected one in the economics of innovation. This chapter, written for a handbook on innovation, provides a historical and comparative perspective on diffusion that looks at the broad determinants of diffusion, economic, social, and institutional, viewed from a microeconomic perspective. A framework for thinking about these determinants is presented along with a brief nontechnical review of modeling strategies used in different social scientific literatures. It concludes with a discussion of gaps in our understanding and potential future research questions.