Abstract
Cities accommodate population growth both through the densification of their existing footprints and through their expansion into their rural peripheries. In recent decades, most population growth has been accommodated through urban expansion. In the Global South, where most urban population growth now takes place, urban expansion occurs in a haphazard, unplanned manner, making cities less productive, less inclusive, less resilient, and less sustainable. This editorial provides a non-technical introduction to the subject of urban expansion, providing the context for the articles in the special issue. The pace and dimensions of global urban expansion and the political contexts in which it occurs are explored and explained. The adverse consequences of unplanned urban expansion are considered, and a case is made for the public sector to assume a key role in preparing rapidly growing cities for their inevitable growth. Seven principles are provided for the public sector to guide planned urban expansion. A set of questions is presented that together form a comprehensive future research agenda on urban expansion.