This book provides the first unifying treatment of the range of economic reasons for the clustering of firms and households. Its goal is to explain further the trade-off between various forms of increasing returns and different types of mobility costs. Although referring to agglomeration as a generic term is convenient, it should be noted that the concept of economic agglomeration refers to distinct real world situations. The main focus of the treatment is on cities, but it also explores the formation of agglomerations, such as commercial districts within cities, industrial clusters at the regional level, and the existence of imbalance between regions. The book is rooted within the realm of modern economics and borrows concepts from geography and regional science, which makes it accessible to a broad audience formed by economists, geographers, regional planners, and other scientists. It may be used in coursework for graduate students and upper-level undergraduates.