In a period when social science disciplines were establishing themselves as separate knowledges, the Harris and Ullman (1945) article stands out as a multidisciplinary project. Harris and Ullman were geographers who chose to publish their article in a leading sociological journal. The article is widely remembered for its famous composite diagram showing concentric, sector and multiple nuclei models of cities. Of these three basic models of the internal structure of cities, one was from sociology, another was from land economics, and they added their own geographical proposal. And this multidisciplinarity is very proper: cities are inherently multifaceted, therefore the idea that any one ‘specialist’ social science can encompass this complexity is foolhardy. This volume comes out of the same disciplinary stable as Harris and Ullman and navigates through a similar contemporary multidisciplinary subject matter. My chief guiding lights have been an economist (Jane Jacobs), a planner (John Friedmann) and two sociologists (Saskia Sassen and Manuel Castells), aided and abetted by many fellow geographers. In these more ‘global times’, I have personally found a curious geographical division in the multidisciplinarity: presentations of the ideas upon which this book is based in continental Europe have been largely to geography audiences, the few papers delivered in Britain have been largely to planners, and nearly all presentations in the United States have been to sociologists. No matter, wherever and whatever the urban researchers, all are cognizant of the need to work across disciplines and not within them. This book is written in precisely this spirit.