What, in terms of space, is an urban area as part of a whole city? And, how do the areas aggregate to form the whole? This book proposes that the discontinuities between urban areas can be typically considered as what we will call fuzzy boundaries that arise from the way space is structured internally and how this relates to the external structure of space. Fuzzy boundaries can be effective in supporting functional differentiation of areas, but do not depend on the area being either spatially self contained or geometrically differentiated, or having clear spatial limits. It is the syntactic relations of all individual spaces and their multi-scale contexts that account for the spatial definition and aggregation of urban areas.