Relatively permanent and highly organized centre of population, of greater size or importance than a town or village. The first cities appeared in Neolithic times when the development of agricultural techniques assured surplus crop yields large enough to sustain a permanent population. Ancient Greece saw the creation of the city-state, a form also important in the emergence of the Roman empire as well as the medieval Italian trading centers of Venice, Genoa, and Florence. After the Middle Ages, cities came increasingly under the political control of centralized government and served the interests of the nation-state. The Industrial Revolution further transformed city life, as factory cities blossomed rapidly in England, northwestern Europe, and the northeastern U.S. By the mid-20th century, 30–60% of a countrys population might be living in its major urban centers. With the rise of the automobile came the growth of suburbs and urban sprawl, as factories, offices, and residences erected in earlier periods became aged and obsolete. Today many cities suffer from lack of adequate housing, sanitation, recreational space, and transportation facilities, and face problems of inner-city decay or burgeoning shantytowns. Local governments have sought to alleviate these problems through urban planning.