The frontier is undergoing a resurgence as a productive spatial trope in urban geographical research. This article provides a critical overview of this “frontier moment” and contextualises it within the history of geography and allied disciplines. Building on this tradition, and drawing on the studies collected together in this special issue, we argue that the frontier is a powerful conceptual lens that can generate new insights into the 21st century urban. The paper develops a new typology of urban frontier thought: urbanizing frontiers; peripheral frontiers; violent frontiers; and capitalist frontiers. In light of the troubled history of frontier thought, it proposes two principles for critical and reflexive scholarship on urban frontiers. First, this scholarship should challenge the colonial myth of empty land, or terra nullius, by making visible the agency of those social actors situated “beyond” the frontier. Second, it should problematise assumptions that frontier spaces signify an inevitable and all-encompassing logic of expansion by recognising the continual reproduction of “outsides” that exceed capitalist urbanization.