This dissertation concentrates on the size distribution of cities. From an empirical perspective, it is shown that Zipf's law for cities holds for the upper tail of the distribution and that the overall distribution is the Double-Pareto-Lognormal distribution. From a theoretical perspective, the dissertation builds a dynamic general equilibrium model of random urban growth with endogenous city formation, which explains the empirical finding of a Double-Pareto-Lognormal city size distribution.