Borderlands history is a burgeoning field that churns out dozens of important monographs every year. Despite their individual insights, these works collectively lack conceptual glue to hold them together as a field. Using US-Mexican border historiography as a case study, this article synthesizes recent scholarship to extract an emerging but as yet not fully articulated methodology that unites much of the field's scholarship of the last decade. Regardless of their individual time, place, or subject matter emphases, the works considered here share common starting questions that inquire what borders are present in any historical context and what their origins and consequences are. The keystone of the methodology is a dialect through which borderlands historians understand borders to be constructed, contested, reconstructed, and ultimately unfinished and unstable. Borderlands historians extend their analysis to include the mental categories with which people have historically made sense of their worlds, and they employ innovative narrative structures that integrate large and small spatial and geographical scales into coherent stories. As a research field seeking to explain the origins and evolution of national and other borders, borderlands history explores one of the most important developments of modern history. As a teaching field that introduces students to the history of boundary making and crossing, borderlands history helps prepare them for twenty-first-century world citizenship.