Process tracing is a theory-based evaluation approach. Based on the formulation of a process theory-of-change, it collects evidence to ascertain how the intervention unfolded in a single case and whether it plausibly contributed to change in outcomes. Often described as a qualitative method, process tracing can in fact rely on a diversity of qualitative and quantitative methods. Particularly useful to evaluate complex interventions, it addresses the questions of “under what conditions, how, and why” an intervention worked, rather than how much impact it produced.