METROPOLITANIZATION: THE MAIN TREND OF AMERICAN URBAN DEVELOPMENT IN THE TWENTIETH CENTURY Wang Xu (65) Many scholars concluded that large American cities were on the wane when the pop ulation of suburban areas first exceeded that of urban areas in the 1970s. By analyzing the history of American metropolitan districts, one finds that the mai n trend in American urbanization in the 20th century is the development of metro politan districts, especially large ones. The horizontal expansion centered on m etropolises, enlarging their urban and suburban areas continually and raising th e level of urbanization uninterruptedly, can also be called metropolitanization.