This article focuses on the transnational diffusion of social movement ideas. How do social movements in one country or region of the world diffuse to another country or region? How do social movement participants learn about other movements and ideas in faraway countries and mobilize around these same ideas? What are the channels and mechanisms of diffusion? These are the research questions the article addresses. I draw on the theoretical literature on social movements and recent research on diffusion to explore these questions. The discussion is applied to two recent cases of diffusion: The diffusion of the Transition movement from the United Kingdom to the United States and the diffusion of the solidarity economy movement from Latin America, France, and Canada to the United States. I argue that a combination of relational, nonrelational, and mediated diffusion explain the spread of the transition and solidarity economy movements. The article contributes to the literature on social movements in two ways. First, it contributes to the recent efforts to better understand the process of diffusion of social movements and their ideas.