This book draws on the experience of a team of the world's finest urban historians to answer a perennial question: why has the urbanised world always generated a small number of cities which are far larger than all the others, and which appear to dominate whole countries, continents, or even the whole world? Two thousand years ago, an area covering most of Europe, North Africa and the Middle East was ruled from Rome. From 1650 to 1914, London was the new Rome, capital of the largest empire the world had ever seen and the financial centre of the world's largest industrial economy. Now, New York is the major world city, but it has already been overtaken in size by Mexico City, Sao Paulo, and other third world cities. Moving through time, these cities look very different, yet their size and dominance suggest that they have a timeless function which means that, whatever their defects and problems, the giant cities will always be with us.