Urban systems are geographical locations with a high concentration of human activity and interactions nested within multiscale interconnected social, engineered, and natural systems that have an influence on human and planetary well-being at several spatial (local to global) and temporal scales. They include cities, towns, and villages. Urban systems range in size from small neighborhoods to large metropolises. They may be located near water bodies or not. Cities exist on all continents except Antarctica, with the exception of Australia and New Zealand. The largest city in the world is Shanghai, China. The most populous country in the world is also called China. The other major population centers in Asia are Tokyo, Osaka, Seoul, and Bangkok. In Europe, they include Moscow, London, Paris, Berlin, Rome, Madrid, and Istanbul. In North America, they include Toronto, Mexico City, Los Angeles, San Francisco, Houston, Chicago, and Washington, D.C. Almost half of the people on Earth live in urban areas. This number is expected to increase to more than two thirds by 2050. The main reason for this rapid growth is that many countries with low or no pollution levels already have large numbers of urban residents. Pollution is often the result of poor planning and construction techniques used in developing countries with little regulation when constructing buildings or repairing roads. In more developed countries, pollution comes from the use of motor vehicles as well as industrial activities.