This document summarizes different theories of social movements from 1900 to the present. It outlines major theorists from this time period including LeBon, Olson, Tilly, and Tarrow who developed theories of collective behavior, collective action, resource mobilization, and political process respectively. It also categorizes American history into four epochs of social and political challenges: the Gilded Age, Progressive Era, Postwar period, and Postmodern era. Major social movements and political contexts are listed for each epoch.