This chapter discusses the insights yielded by studies examining inter- and intra-movement diffusion, with an eye toward understanding what diffuses, through which channels, and the catalysts to the diffusion process. It also discusses the relationship between cycles of protest and diffusion, and the still nascent tradition of understanding the impact of diffusion on other social processes. Social movement studies demonstrate that diffusion processes involve a range of different actors, networks and mechanisms, and they highlight the role of agency in translating, transforming, and adapting innovations invented by others. It concludes with concrete suggestions for future research.