This paper models and examines empirically the evolution of cities in an economy. Twentieth century evolution in the USA is characterized by parallel growth of cities of different types and on-going entry of new cities, together maintaining a stable relative size distribution of cities. Each type of city has a particular industrial composition and good(s) it specializes in and corresponding equilibrium size. This evolution is modeled in an economy with exogenous population growth and endogenous human capital accumulation. Within cities, there are knowledge spillovers as well as scale externalities. Individual city sizes grow with human capital accumulation; and cities grow in number if national population growth is high enough. Different types of cities grow in parallel in size and human capital accumulation. However, per capita income and human capital levels differ across city types by production process and benefits of human investments and spillovers, so there is observed inequality across cities among otherwise identical individuals.