This volume should be required reading in all political science research design seminars and for all political scientists exploring how to contribute to scholarship in the discipline. Subnational research has often been relegated to one of two types: (1) a point of theoretical or empirical focus whose causal theories and empirical findings are limited in scope and (2) a way to increase the number of cases and the robustness of empirical tests of theoretical models (but only in combination with tests conducted at other—usually higher—levels). Although these reasons are in themselves justifications for subnational analysis, Agustina Giraudy, Eduardo Moncada, and Richard Snyder show how taking research to the subnational level can lead to new and improved theories, surprising substantive findings, and important methodological innovations that stretch far beyond subnational units and their national borders.

Across all subfields in the discipline, political scientists aim to identify and fill theoretical, empirical, and methodological gaps. Scholars can focus on building new theories to explain newly observed or known empirical phenomena, adapting existing theories to explain newly observed or known empirical phenomena, clarifying the causal mechanisms in existing theories about known empirical phenomena, distinguishing between alternative arguments explaining the same empirical phenomena, or resolving contradictory causal processes explaining the same empirical phenomena. Scholars can focus on identifying new empirical phenomena or data to test new theories, finding new empirical phenomena or data on which to retest existing theories, bringing new empirical phenomena or existing empirical data to bear on untested but existing theories, or identifying and explaining puzzling empirical phenomena or contradictory data. They can focus on inventing new methodological approaches or improving and adapting existing ones to test new theories, inventing new methodological approaches or improving and adapting existing ones to retest existing theories or test untested ones, inventing new methodological approaches or improving and adapting existing ones to analyze newly observed empirical phenomena or data, or inventing new methods or improving and adapting existing ones to analyze existing empirical phenomena or data.

With these possible research contributions in mind, most scholars would probably identify subnational research as bringing only limited opportunities for major contributions. However, Giraudy, Moncada, and Snyder explain how subnational research can reveal crucial theoretical, substantive, and methodological gaps worthy of our attention.

In the theoretical realm, the editors note that subnational analysis can lead to the creation of new theories that transcend the cases for which they were developed. Specifically, subnational research has produced new theories that consider the interaction of different levels of government, including bottom-up theories that explain how subnational factors, actors, and institutions affect national ones and top-down theories that explain how national factors, actors, and institutions affect subnational ones. Theories that explain reciprocal relationships between the subnational and national factors, actors, and institutions should also be produced, because it is unlikely that the relationship between these levels is unidirectional. The editors also explain how adaptation of national theories to explain these phenomena can sometimes work well but can also, instead, reveal important theoretical limitations in our understanding of their operation in the subnational world. Rather than relegating the subnational realm to a lesser position, the fact that so much political activity occurs at this level rather than at national levels reveals the need for and importance of subnational-oriented theoretical innovations that can be applied across different national cases.

These theoretical observations are not new; scholars of multidimensional theories or adapted national-to-subnational theories have made them before. However, the editors support their observations with evidence in the form of chapters by invited collaborators. For those seeking to understand the importance of multilevel analysis, these chapters offer new, multilevel theories. For example, thanks to their innovative top-down theoretical approach, Guillermo Trejo and Sandra Ley explain how national–subnational political interactions affect criminal violence against local politicians in Mexico. Caroline Beer also breaks new theoretical ground when she explains how state political interdependence interacts with national politics to affect the diffusion of abortion and violence against women laws in Mexico. For those seeking to understand the limitations of national theories, other chapters show how national theories fail to explain important subnational dynamics that can affect national factors, actors, and institutions. For example, using matched-case comparisons Gavril Bilev uncovers how and why subnational political regimes rarely reproduce national-level dynamics, focusing on the case of subnational governments across Russia to make this point. Similarly, using carefully matched cities, Meg Rithmire explains the shortcomings of top-down, national-led explanations for the emergence of local property rights regimes in China and builds a multilevel theory that explains how the timing of national measures with local political conditions affected outcomes. In this way, the authors support their point that subnational analysis can result in theoretical contributions that fill crucial theoretical gaps in our understanding of the world.

One of the most important contributions of the volume is the chapters’ focus on the important methodological opportunities created by subnational research. Mixed-methods research design advocates have long relied on subnational analysis to bolster the robustness of their empirical tests and the causal inferences drawn from them. However, Giraudy, Moncada, and Snyder join this long-held observation by noting four additional methodological contributions that can come from subnational analysis. The editors and their contributors note the important role that subnational research can play in enabling the identification of causal mechanisms. The presence of subnational units creates opportunities to match units and assign them to treatment and control groups. As long as the treatments are exogenous to unit selection and the units are independent from one another, matched-case subnational analysis of this type can create a quasi-experimental research design that allows scholars to control for confounding factors and isolate the impact of any causal factors under examination. For example, Prerna Singh is able to take advantage of a matched-case strategy to develop new theory about how variation in subnational solidarity explains variation in social development trajectories across Indian states. Thanks to their matched-case comparisons, Sunila S. Kale and Nimah Mazaheri are able to explain how the presence and activities of civil society organizations affect the production and delivery of policies and programs aimed at local indigenous communities.

The availability of subnational research can encourage scholars to clarify the level and type of unit at which their theories operate and thus the optimal level and type of unit for testing them. Hillel David Soifer explains that theories can be unit independent and apply to any unit of analysis, unit specific to certain units of analysis, and unit limiting to only certain types of units. He notes that scholars should spend time explaining the relevant units of analysis to which their arguments can be applied and thus to all the levels at which the analysis could be conducted. Moreover, scholars should also explicitly note to which units and levels of analysis their theory does not apply. This can help scholars leverage multiple levels and units of analysis to test their arguments. In her follow-up chapter, Ana Arjona builds on this discussion and notes that identifying the optimal level and units of analysis also requires unpacking the locus of decision-making within the causal process, specifically identifying which contextual factors matter most to actors whose decisions affect the causal process under examination. In the political realm, citizens, voters, rebels, politicians, elites, and rulers are embedded within nations, regions, states, localities, and neighborhoods, with the context most relevant for their decision-making in the causal process under examination directing scholars to the optimal level and thus unit of analysis. The possibility of subnational analysis thus forces scholars to clarify the context within which the actors, factors, and institutions under examination operate in their causal arguments, helping clarify both causal logics and optimal levels and units for empirical analysis.

Subnational research also creates opportunities to methodologically account for unit interdependence. The danger of subnational analysis is that subnational units are usually treated as independent from one another, something that is unlikely to be the case because they are housed within the same national systems. However, Giraudy, Moncada, and Snyder also stress that subnational analysis offers unique opportunities for theorizing about and substantively examining the basis of unit interdependence. To make this point, the editors call on the insights of Imke Harbers and Matthew C. Ingram, who explain that subnational units might be interdependent in one of three or more ways. Units might be clustered in some unexplained way, known as spatial clustering (spatial error models); if unaccounted for, such clustering can bias results and interfere with causal inferences. Units might also be interdependent in that the causes of an outcome in one unit might affect the causes of the outcome in other units, known as spillover effects (captured in spatial X models). Units might also be interdependent in that the outcome of causes in a unit affects the outcome of causes in other units, known as diffusion effects (spatial lag models). Although theories that highlight spillover effects or diffusion have been developed and tested at national and subnational levels, Giraudy, Moncada, and Snyder remind us that subnational analysis is particularly appropriate for developing and testing such unit interdependence. The invited chapters also provide additional illustrations of these points.

The editors and contributors pay close attention to explaining and illustrating how subnational analysis can open avenues for innovative theoretical and methodological approaches that can substantively advance knowledge. That is precisely why I would recommend this volume to scholars of both comparative politics and political economy and to those interested in conducting political science inquiry more broadly.