This article assesses the utility and validity of a qualitative method called process tracing, and then reports the logic of inference called concatenation. It also investigates the concept of causal mechanisms and its relationship to process tracing. Process tracing, which can assist both causal inference and causal explanation, is defined as a mode of causal inference based on concatenation, not covariation. Its explanatory value depends on the case for mechanism-based explanations. In addition, process tracing tries to understand the importance of the distinction between causal inference and causal explanation. It is noted that process tracing is an extremely valuable tool of both inference and explanation. Claims about its accomplishments and value should be commensurately tempered.