Today, for the first time in human history, more than half of humanity lives in cities. Yet until about 500 years ago, cities were few and fragile. This book focuses on history’s major exception: the Mediterranean in classical antiquity. Overall urbanization rates there reached around 20 percent, higher than Europe would see again until the nineteenth century. The force behind ancient Mediterranean urbanization was economic: control over the surplus from surrounding agricultural land and, for the largest cities, privileged access to long-distance trade. Yet ancient cities were not self-sufficient: the more they grew, the more taxes they had to levy; the more food, water, stone, metals, and luxury goods they had to import; and the more slave labor they needed to offset high urban mortality from famine, disease, fire, and natural disasters. Ancient cities became the hubs of hegemonies and empires—a highly leveraged arrangement that made them vulnerable to sudden shocks and military decline. They collapsed much faster than they had been built: in three centuries, Rome’s population declined from over a million to just about 10,000. Today’s global cities face similar vulnerabilities, and one wonders whether future historians will write about them in the same way.