Urbanization is one of the dominant demographic trends of our time. In 1900, 150 million people lived in cities. By 2000, it was 2.9 billion people, a 19-fold increase. By 2007 more than half of us will live in cities -- making us, for the first time, an urban species. In 1900 there were only a handful of cities with a million people. Today 408 cities have at least that many inhabitants. And there are 20 megacities with 10 million or more residents. Tokyo's population of 35 million exceeds that of Canada. Mexico City's population of 19 million is nearly equal to that of Australia. New York, Sao Paulo, Mumbai (formerly Bombay), Delhi, Calcutta, Buenos Aires, and Shanghai follow close behind. Cities require a concentration of food, water, energy, and materials that nature cannot provide. Concentrating these masses of materials and then dispersing them in the form of garbage, sewage, and as pollutants in air and water is challenging city managers everywhere. Most of today's cities are not healthy places to live. Urban air everywhere is polluted. Typically centered on the automobile and no longer bicycle- or pedestrian-friendly, cities deprive people of needed exercise, creating an imbalance between caloric intake and caloric expenditures. As a result, obesity is reaching epidemic proportions in cities in developing as well as industrial countries. With more than 1 billion people overweight worldwide, epidemiologists now see this as a public health threat of historic proportions -- a growing source of heart disease, high blood pressure, diabetes, and a higher incidence of several forms of cancer. The evolution of modern cities is tied to advances in transport, initially for ships and trains, but it was the internal combustion engine combined with cheap oil that provided the mobility of people and freight that fueled the phenomenal urban growth of the twentieth century. As the world urbanized, energy use climbed.