Paul Pierson's “Not Just What, but When: Timing and Sequence in Political Processes” is a fitting centerpiece for this symposium. The article takes up themes that have occupied students of comparative politics for some time now and moves the debate decisively forward.For earlier efforts to assess the role of sequencing and timing in politics, see for example, Harsanyi's discussion of “static” and “dynamic” explanation (J. Harsanyi, “Explanation and Comparative Dynamics in Social Science,” Behavioral Science 5 (1960): 136–45); also Verba's early attempts within the context of modernization theory to specify more precisely a “sequential model” that linked outcomes within particular countries to the sequence in which they encountered a set of putatively common challenges (Sidney Verba, “Sequences and Development,” in Leonard Binder et al., Crises and Sequences in Political Development [Princeton, NJ: Princeton University Press, 1971], 283–316). Tilly has dealt with related issues of temporality and ordering; see Charles Tilly, “Future History,” Theory and Society 17 (1988): 703–12; Tilly, Big Structures, Large Processes, Huge Comparisons (New York: Russell Sage Foundation, 1984); and Tilly, “The Time of States,” Social Research 61 (1994): 269–95. Schmitter and Santiso have also addressed these topics (Philippe C. Schmitter and Javier Santiso, “Three Temporal Dimensions to the Consolidation of Democracy,” International Political Science Review 19 (1998): 69–92). And of course, Pierson's agenda picks up on themes long advocated by the editors of this journal to take temporality seriously (Karen Orren and Stephen Skowronek, “Beyond the Iconography of Order: Notes for a ‘New' Institutionalism,” in The Dynamics of American Politics, eds. Lawrence C. Dodd and Calvin Jillson (Boulder, CO: Westview, 1994). Pierson's overall message is that social phenomena are often better captured in “moving pictures” that situate a given outcome within a broader temporal framework than in “snapshots” based on cross-sectional data. He constructs a convincing case for this proposition and along the way he also makes progress in rendering the ubiquitous but vague concept of path dependence more useful.For other treatments of this issue see Stephen D. Krasner, “Sovereignty: An Institutional Perspective,” Comparative Political Studies 21 (1988): 66–94; Timur Kuran, “The Tenacious Past: Theories of Personal and Collective Conservatism,” Journal of Economic Behavior and Organization 10 (1988): 143–71; Douglass C. North, Institutions, Institutional Change and Economic Performance (New York: Cambridge University Press, 1990); and James Mahoney, “Uses of Path Dependence in Historical Sociology,” unpubl. ms., Brown University, 1999.