Urban structure is increasingly characterized by decentralization, dispersion, and multiple employment centers. Much is known empirically about such patterns, and about how the interplay between agglomerative and dispersive forces generates spatial structures that are complex and prone to multiple equilibria and dynamic path-dependence. These forces operate at different spatial scales; many entail unpriced interaction, and external scale economies deriving from product differentiation and endogenous technical change appear particularly important. Because these forces interact in complex ways, inefficiencies in urban structure are resistant to simple policy interventions.