This article develops a framework for the causal analysis of critical events in case study research. A critical event is defined as a contingent event that is causally important for an outcome in a specific case. Using set-theoretic analysis, this article offers definitions and measurement tools for the study of contingency and causal importance in case study research. One set of tools consists of guidelines for using theoretical expectations to arrive at conclusions about the level of contingency of events. Another set of tools are guidelines for using counterfactual cases to determine the extent to which a given event is necessary and sufficient for a particular outcome in an individual case. Examples from comparative and international studies are used to illustrate the framework.