The paper by Edwin S. Mills presents an aggregative model to explain the sizes and structures of urban areas, focusing on the market responses to production and income opportunities. The model emphasizes the importance of input substitution and technology in explaining urban structure, particularly the variation in factor prices and land use across different parts of a city. The author argues that the existence and size of cities are driven by the availability of favorable land and the increasing returns to scale in certain activities, such as goods production and transportation. The model assumes that land is homogeneous and production functions have constant returns, but relaxes these assumptions to justify the formation of cities. The paper also discusses the implications of these assumptions for the distribution of land use, population density, and transportation costs. The solution to the model reveals that the central business district (CBD) will be circular, and the distribution of land use and population density will depend on the efficiency parameters and factor prices. The model provides insights into the dynamics of urban development, including the impact of transportation costs and the optimal allocation of land for different activities.The paper by Edwin S. Mills presents an aggregative model to explain the sizes and structures of urban areas, focusing on the market responses to production and income opportunities. The model emphasizes the importance of input substitution and technology in explaining urban structure, particularly the variation in factor prices and land use across different parts of a city. The author argues that the existence and size of cities are driven by the availability of favorable land and the increasing returns to scale in certain activities, such as goods production and transportation. The model assumes that land is homogeneous and production functions have constant returns, but relaxes these assumptions to justify the formation of cities. The paper also discusses the implications of these assumptions for the distribution of land use, population density, and transportation costs. The solution to the model reveals that the central business district (CBD) will be circular, and the distribution of land use and population density will depend on the efficiency parameters and factor prices. The model provides insights into the dynamics of urban development, including the impact of transportation costs and the optimal allocation of land for different activities.