Scholars invoke the terms “millenarian,” “millennialist,” and “apocalyptic” to refer to movements and sects that: embrace ideologies positing the (typically traumatic) end of one epoch; promise relief from the sufferings of this world and its dystopian prospects; and offer purported salvation in life after death, a new “golden age,” a “heaven on earth,” or a realized utopian social order. The strikingly broad range of movements that adopt such ideologies includes peaceful conversionist movements and militant religious social movements undertaking “holy war,” anti-colonial movements, agrarian movements, and modern revolutionary political movements. Moreover, movements vary widely in their scale of organization and significance – from those that attract little notice beyond their participants to movements within early Christianity, historical and contemporary Islam, and the Protestant Reformation that have been civilizational, even world-historical, in their consequences. Despite the ancient Middle Eastern origins of apocalypticism and millennialism, non-Western religions and ideologies – for example, Buddhism – have on occasion provided independent inspiration for such movements. Sometimes too, non-Western movements (for example, the nineteenth-century Taiping Rebellion in China) have synthesized local cultural materials with millenarian Christian ideas. In short, rich veins of case study and comparative research document that millenarian and apocalyptic movements vary in ideology, organizational form, scale, and trajectory. The broad range of cases and their varying significance pose two important challenges – how to theorize alternative types of such movements and how to analyze the trajectories of groups in relation to social theories and history.