Political scientists frequently use qualitative evidence to support or evaluate the empirical applicability of formal models. Despite this widespread practice, neither the qualitative methods literature nor research on empirically evaluating formal models systematically address the topic. This article makes three contributions to bridge this gap. First, it demonstrates that formal models and qualitative evidence are indeed frequently combined in current research. Second, it shows how process tracing can be as important a tool for empirically assessing models as statistical testing, because models and process tracing share a common focus on understanding causal mechanisms. Last, it provides new guidelines for using process tracing that focus on issues specific to the modeling enterprise, illustrated with examples from recent research.