This chapter is concerned with the presentation and analysis of regularities in two best-documented examples of early, independent urban societies. It seeks to provide as systematic a comparison as the data permits of institutional forms and trends of growth that are to be found in both of them. The term "Urban Revolution" implies a focus on ordered, systematic processes of change through time. Hence the identifying characteristics of the Urban Revolution need to be more than loosely associated features, whose functional role is merely assumed and which are defined in terms of simple presence or absence. A basic question relating to the possibility of discontinuities in the causally interrelated web of changes that constitutes the Urban Revolution concerns the rate and duration of the transformation itself. There is a broad change in the character of the data with the onset of the Urban Revolution.