The histories of all modern scientific and intellectual fields are marked by dynamism. Yet, despite a welter of case study data, sociologists of ideas have been slow to develop general theories for explaining why and how disciplines, subfields, theory groups, bandwagons, actor networks, and other kindred formations arise to alter the intellectual landscape. To fill this lacuna, this article presents a general theory of scientific/intellectual movements (SIMs). The theory synthesizes work in the sociology of ideas, social studies of science, and the literature on social movements to explain the dynamics of SIMs, which the authors take to be central mechanisms for change in the world of knowledge and ideas. Illustrating their arguments with a diverse sampling of positive and negative cases, they define SIMs, identify a set of theoretical presuppositions, and offer four general propositions for explaining the social conditions under which SIMs are most likely to emerge, gain prestige, and achieve some level of institutional stability.