This chapter examines the functions of and prospects for large metropolitan areas (MAs) in the United States. It argues that the high cost of transporting people and goods is a necessary, but not sufficient condition for MAs. Economies of scale and scope and the technical ability to substitute structures for land provide cost advantages to firms located in MAs. The factors that limit the sizes of the largest MAs include the size and geography of the country, the limited demand for commodities and services produced in the MA, congestion and pollution, and social issues such as crime, homelessness, poverty, illegitimacy, racial tensions, and other forms of alienation that increase with MA size. Because of these limiting factors, the largest US MAs are expected to grow at slower rates than the US population in coming decades, whereas suburbs will continue to experience rapid growth.