Since ancient Greece’s “megalopolis,” the concept of vast cities has loomed in the urban discourse. A century ago, English planner Patrick Geddes warned about a growing imbalance between traditional society and ever-larger conurbations, an anxiety that Lewis Mumford later invoked to predict that urban hubris would inevitably collapse of its own weight. In 1961, by contrast, the geographer Jean Gottman surveyed the interconnected agglomeration stretching from Washington, D.C. up the east coast of the United States to the cities of southern New England, and more optimistically highlighted this new urban form’s governance potential. Today, the question is not whether urbanism will arrive at the scale that so concerned Geddes and Mumford yet engaged Gottman—it already has. Commentators and scholars increasingly recognize that vast polycentric urban regions are displacing cities and metropolitan areas as the locus of modern growth and development. Eleven distinct urban-centered megaregions, for example, concentrate the bulk of the population and economic activity in the United States, and similar clusters are coalescing globally, from Greater Tokyo, Beijing, and the Pearl River Delta, to the arc from São Paulo to Rio de Janeiro, to the European urban spine running from London to Milan. Grappling with the phenomenon’s legal-institutional dimensions—whether and how to formalize governance to match this scale—requires more than transposing the descriptive and normative discourse on metropolitanism. Crosscurrents around fragmentation, efficiency, inequality, and democratic legitimacy refract, but there are distinct arguments for fostering governance—and equally particular concerns to anticipate—with the rise of the megalopolis.