Process tracing is a research method for tracing causal mechanisms using detailed, within-case empirical analysis of how a causal process plays out in an actual case. Process tracing can be used both for case studies that aim to gain a greater understanding of the causal dynamics that produced the outcome of a particular historical case and to shed light on generalizable causal mechanisms linking causes and outcomes within a population of causally similar cases. This article breaks down process tracing as a method into its three core components: theorization about causal mechanisms linking causes and outcomes; the analysis of the observable empirical manifestations of the operation of theorized mechanisms; and the complementary use of comparative methods to enable generalizations of findings from single case studies to other causally similar cases. Three distinct variants of process tracing are developed, illustrated by examples from the literature.