Although its origins lie in cognitive psychology, process tracing has become a “signature method” of many qualitative and multi-method scholars in political science and beyond. Simply put, process tracing is about analyzing the chains of events that lead to an outcome of interest. More formally, the editors of this recent volume, Andrew Bennett and Jeffrey T. Checkel, define process tracing as the “analysis of evidence on processes, sequences and conjunctures of events within a case for the purposes of either developing or testing hypotheses about causal mechanisms that might causally explain the case” (7). As “process tracers” strive at producing sound causal inferences based on a rich and detailed account of one or many cases, they participate in a movement that emphasizes the importance of sound explanation in political science (Daigneault and Béland, Reference Daigneault and Béland2014; Parsons, Reference Parsons2007).

Not so long ago, the issue faced by scholars was to justify their use of process tracing as a legitimate research method in and of itself, in particular after the publication of Designing Social Inquiry: Scientific Inference in Qualitative Research (King, et al., Reference King, Keohane and Verba1994), which construed process tracing merely as a way to increase the number of observable implications of a theory. Convincing responses to this challenge came from many quarters, including from Henry Brady and David Collier (Reference Brady and Collier2004). Today, the question is not whether process tracing can be used to explain important events, decisions, or outcomes—it definitely can—but rather how to use this method transparently, rigorously and effectively.

Process Tracing: From Metaphor to Analytic Tool aims at “making process tracing real” (xii) by showing in a concrete and operational way how to apply this method well. After a brief discussion of the philosophical and conceptual underpinnings of process tracing, a discussion addressing issues such as the nature of causal mechanisms, methodological individualism, generalizability, Bayesian theory and evidentiary claims, the editors make three arguments with respect to “good” process tracing: 1) it should be consistent with a mechanism-based understanding of social reality; 2) it should rely on pluralism to reconstruct causal sequences and analyze context; 3) it should take seriously both equifinality (that is, the fact that multiple combinations of causes can lead to an outcome) and alternative explanations. Then the editors put forward ten best practices, which serve not only to guide process tracers but also to evaluate their work. These guidelines for process tracers undoubtedly are the crux of Bennett and Checkel's contribution. Naturally, the best practices are amply discussed in the book, but only a list is provided for the purpose of the present review (21):

• • Cast the net widely for alternative explanations.

• • Be equally tough on the alternative explanations.

• • Consider the potential biases of evidentiary sources.

• • Take into account whether the case is most or least likely for alternative explanations.

• • Make a justifiable decision on when to start.

• • Be relentless in gathering diverse and relevant evidence, but make a justifiable decision on when to stop.

• • Combine process tracing with case comparisons when useful for the research goal and feasible.

• • Be open to inductive insights.

• • Use deduction to ask “if my explanation is true, what will be the specific process leading to the outcome?”

• • Remember that conclusive process tracing is good, but not all good process tracing is conclusive.

The remaining contributions to this volume are devoted to operationalizing, illustrating and discussing these ten best practices, without neglecting the larger philosophical, conceptual and methodological issues pertaining to process tracing. In Part I of the volume, chapters 2 to 7 serve to “make process tracing real” in relation to various subfields, research programs and topics, namely ideational theories (Alan Jacobs), international institutions (Jeffrey Checkel), European integration (Frank Schimmelfennig), comparative politics (David Waldner), the end of the Cold War (Matthew Evangelista) and civil wars (Jason Lyall). The three chapters of Part III address what the editors characterize as the “research frontier” of process tracing. Chapters 8 and 9 explore the relationship between process tracing and quantitative and multi-method research (Thad Dunning), as well as its role in the interpretive tradition (Vincent Pouliot). In chapter 10, the editors take a step back to reflect upon and assess the various contributions and fine-tune their arguments. Finally, a technical appendix by Andrew Bennett addresses the formalization of process tracing through Bayesian analysis.

This much-needed book is largely successful in providing operational guidance as to how to conduct sound process tracing. Graduate students and junior scholars who plan to use process-tracing methods would definitely benefit from reading this book from cover to cover, if only to sensitize themselves to the important issues they should consider. The ten best practices appear sound and justified. Furthermore, even those who do not plan to use process tracing could benefit from reading individual chapters, such as Jacobs's excellent chapter on policy ideas, for what these chapters have to teach about general empirical research methods or for their own sake. The ten contributions to this book, written by leading scholars in the field, are all very relevant and of a consistently high quality, which is admittedly an achievement for an edited volume like this. Moreover, all the chapters contain rich empirical material, are insightful and are well written. Additionally, the editors have selected contributors so as to ensure a certain level of diversity in terms of subfields and research programs, epistemological perspectives (scientific realism versus interpretivism) and methodological proclivities (deductive versus inductive). This careful selection results in a fruitful balance; though the contributors generally adhere to the editors' best practices, they also openly discuss them and, in some cases, propose worthwhile amendments and revisions such as “efficient process tracing” (Schimmelfennig), “the completeness standard” (Waldner), “additional best practices” (Lyall) and “practice tracing” (Pouliot).

However, I was disappointed—but not surprised—by the disproportionate number of contributions from comparative politics and international relations. Indeed, there are important topics outside these two political science subfields to which process tracing could and should be put to work, for instance to analyze important policy outcomes at the domestic level. The volume, then, would have benefited from a few contributions from scholars in related disciplines such as sociology, political psychology, public administration and program evaluation, especially as this book was published in a general social science series (Strategies for Social Inquiry). Moreover, the book contains numerous illustrations of process tracing that, while relevant, might at times overwhelm those reading it from beginning to end; readers could get lost in the book's abundant detail. Because process tracing is context- and data-intensive, unfamiliar readers must take the contributors' word regarding the quality of the process-tracing evidence presented rather than assessing it for themselves. A common empirical case, presented in the introduction or an appendix, on which contributors could draw to illustrate their arguments would have both enabled the reduction of some of this potentially overwhelming detail and served significant pedagogical purposes.

Despite these minor issues, Process Tracing: From Metaphor to Analytic Tool is a great book that makes an excellent contribution to political science. This volume is likely to become a classic for qualitative scholars, a fate that would be entirely deserved.