Johnston, H. & Aarelaid-Tart, A. (2000) "Generations, Microcohorts, and Long-Term Mobilization: The Estonian National Movement, 1940–1991", Sociological Perspectives, 28 pages. doi: 10.2307/1389553. URL: http://spx.sagepub.com/cgi/con. Abstract▼ Generational cohorts are often key collective actors in large-scale mass mobilizations and in movements that develop over long periods. Based on seventy-two biographical interviews, we analyze the generational composition of the Estonian national opposition. Several microcohorts are identified on the basis of experience of historical events and specific political opportunities. These microcohorts clustered around two master collective action frames that, over the long term, shaped the trajectory of the movement into two wings, one claiming a pure Estonian frame for collective action and the other an accommodationist frame. This article traces these dual streams of the national movement during its forty-year trajectory and extrapolates from the analysis several conditions in which generational cohorts are likely to be fundamental causal elements in social movement development.