This chapter on urbanization and growth focuses on modeling and empirical evidence that pertain to a number of inter-related questions. Why do cities form in an economy, with so much of economic activity in countries geographically concentrated in cities? Second, how do different types of cities interact with each other in terms of trade and migration? Given the answers to these questions the chapter turns to growth issues. How does a system of cities evolve under economic and population growth; and how does urban growth intersect with, or even define national economic growth? In growth theory, endogenous growth is based on knowledge spillovers and sharing, and evidence suggests that much of that interaction must occur at the level of individual cities. In the early stages of growth, economic development is characterized by urbanization - a spatial transformation of the economy, where the population moves through migration from an agricultural, rural based existence to one where production occurs in cities of endogenous numbers and size. How do we model that transformation process and what are the key aspects of the transformation? In any static, growth, or development-urbanization context, how do governance, institutions, and public policy affect city formation and sizes, which then in turn affect economic efficiency. Cities require enormous public infrastructure investments which affect urban quality of life, in particular health and safety and commuting and congestion costs. Institutions governing land markets, property rights, local government autonomy, and local financing affect the city formation process and city sizes. And national government policies concerning trade, labor policies and national investment in communications and transport infrastructure affect the shape of the urban system. A final set of questions has to do with where cities locate. What is the effect of history, of climate and of natural resource locations, including rivers and natural harbors, on the location of current urban agglomerations?