Formal theorists frequently use qualitative evidence to validate their theories, and qualitative studies have played an important role in shaping views on the empirical value of some influential formal models. Despite this, the growing literature on qualitative methods pays little attention to the specific issues involved in evaluating formal models, while formal modelers often offer qualitative evidence with insufficient methodological self-consciousness. This paper takes a first step toward constructive engagement between the two literatures. We argue that formal theorists and qualitative researchers share an under-appreciated affinity --- their joint interest in understanding causal mechanisms --- and make a case that process tracing is a valuable tool for empirically evaluating formal models. We then provide guidelines, complemented with numerous examples, that build on the methodological theory and practice of both formal theorists and qualitative scholars in order to establish common ground and help both groups use qualitative evidence more effectively to evaluate formal models.