Over the past thirty years, urban ecomic theory has been one of the most active areas of urban and regional ecomic research. Just as static general equilibrium theory is at the core of modern microecomics, so is the topic of this book - the static allocation of resources within a city and between cities - at the core of urban ecomic theory. An Essay on Urban Ecomic Theory well reflects the state of the field. Part I provides an elegant, coherent, and rigorous presentation of several variants of the mocentric (city) model - as the centerpiece of urban ecomic theory - treating equilibrium, optimum, and comparative statistics. Part II explores less familiar and even some uncharted territory. The mocentric model looks at a single city in isolation, taking as given a central business district surrounded by residences. Part II, in contrast, makes the intra-urban location of residential and n-residential activity the outcome of the fundamental tradeoff between the propensity to interact and the aversion to crowding; the resulting pattern of agglomeration may be polycentric. Part II also develops models of an urbanized ecomy with trade between specialized cities and examines how the market-determined size distribution of cities differs from the optimum. This book launches a new series, Advances in Urban and Regional Ecomics. The series aims to provide an outlet for longer scholarly works dealing with topics in urban and regional ecomics.