This article is a call for volunteers to stand on the shoulders of Gabriel Tarde and Pitirim Sorokin, who dared to theorize the process of diffusion over a wide variety of disciplines. While all of the social sciences and humanities regularly produce case studies of diffusion, theorizing seems paralyzed. This paralysis stems from the ostensible incommensurability of diffusing items; their refusal to hold still in transit; the complexity of their interactions with the cultures, social structures, and media systems in which potential adopters are embedded; the difficulty of reconciling voluntary action and external imposition; and the lack of a disciplinary home.