This article introduces the Polity IIId (“d” is for dates) data set. The Polity IIId project codes the precise dates of changes in political structure identified by Polity III for all independent countries in the international system from 1800 to 1994. By moving from annual measurements of authority and polity characteristics, the Polity IIId data are more appropriate for event-based analysis. The authors discuss the implications of the new data set for event count and event history models of democracy and war, democratization and war, regime type and civil war, and causes of change in political structure.