A century after Herbert E. Bolton brought Anglo-American historians' attention to the so-called “Spanish borderlands,” Danna Levin Rojo and Cynthia Radding have compiled a thorough and far-reaching volume of interdisciplinary scholarship on borderlands that preceded and emerged from Spanish and Portuguese imperial projects. Although similar compilations have been published over the past quarter century, some mentioned by Levin Rojo and Radding in the introduction, none has been nearly as comprehensive as this work (p. 17). The handbook's introduction encapsulates broader trends in borderlands studies since the closing decades of the twentieth century, particularly the shift in focus from interstate borders to intercultural spaces, the attention to Indigenous agency and resilience, and the emphasis on multipolar exchange over binary opposition. Its 34 chapters do not simply showcase these trends; they collectively lay the groundwork for significant conceptual shifts in future works.Part of this book's success derives from its breadth of authors and sites of study. Readers will find remarkable the generational and geographic range of contributors, from emeriti authors of foundational texts to authors of newly published monographs, who are based across the Americas and in parts of Europe. Likewise, while approximately half the chapters pertain to New Spain, where borderlands studies have traditionally focused, there are multiple chapters that address each of the following regions: the Caribbean basin, Brazil, Bolivia, the southern Andes, the Río de la Plata, and the transpacific galleons. With the exception of Ines Županov's chapter on intercultural knowledge production in the Estado da Índia, the focus is thus Iberian borderlands in the Americas and the Philippines.Levin Rojo and Radding organize the chapters into three thematic sections, though conceptual and comparative connections could be made across the handbook. The first eight chapters consider Indigenous borderlands that preceded Iberian arrival and transformed thereafter, with authors paying special attention to food security, landscape transformations, epidemics, military alliances, and information exchange. Together, the chapters situate Iberian colonial projects in deeper Indigenous histories and articulate the ways that Native peoples collaborated with, leveraged, limited, and bore the weight of colonial expansion. The next 18 chapters, broadly grouped together as addressing transcontinental borderlands, appear in four subsections that address trade networks, patterns of identification, knowledge production, and the persistent adaptations of Indigenous Americans amid slave trades, efforts at spiritual conquest, and shifting settlement patterns. Of particular importance are Indigenous traders and commercial exchange, processes of mestizaje and ethnogenesis, cultural production at missions, and struggles for autonomy. The final eight chapters follow the movements of merchants, smugglers, priests, and enslaved peoples across rivers, oceans, and seas where multiple European empires asserted competing jurisdictional claims.One challenge inherent to such a large compilation is the presentation of a conceptual frame that encompasses the diversity of chapter contributions without straying into ambiguity. In the introduction, Levin Rojo and Radding define borderlands as “diffuse spaces produced through historical processes of contestation, adaptation and admixture among different peoples, within specific temporal and geographical frameworks” (p. 1). While some readers may find this definition too broad, it accurately captures the dual elements that the term has assumed over the past half century: a geographical framework seeking to move beyond Eurocentric frontier studies, and a cultural framework focused on exchange and transformation over opposition and isolation. These dual framings are what allow chapters on colonial agents' transimperial and transoceanic networks to fit alongside others focused on Indigenous autonomy, territorialities, and identity formations. More importantly, the authors' definition presents borderlands not as spaces defined by their alterity vis-à-vis administrative centers but instead as defining features of Iberian imperial projects in the Americas and elsewhere.While such a definition of borderlands is admirable in its synthesis and appropriate in its breadth, it understates the paradigmatic shift evident throughout the handbook. Moreover, the notion of “internal borderlands” that appears in the introduction and in numerous chapters reflects the same territorialized political geographies of European empires that the authors seek to eschew. Treating inland spaces instead as networked archipelagos better captures the nodal, polycentric, and route-based spatial structures and political formations that defined imperial and Indigenous geographies alike. Sean McEnroe and Cynthia Radding conceptualize inland archipelagos in their chapters, while other contributors make similar gestures in their discussions of outposts, enclaves, corridors, and island chains (pp. 60, 170). This is not to discount or ignore territorialized land claims or the aspirational colonial cartographies discussed in José Refugio de la Torre Curiel's chapter but to encourage readers to reflect on the persistent spatial tropes embedded in the archival materials that tend to undergird borderlands studies, as Levin Rojo and Radding suggest as well (pp. 14–15). Whatever the framing, readers both new to and entrenched in the field will find this handbook to be a rich collection of works that will provide a conceptual and contextual foundation for years to come.