The urbanization of the world, which is one of the most impressive facts of modern times, has wrought profound changes in virtually every phase of social life. The recency and rapidity of urbanization in the United States accounts for the acuteness of our urban problems and our lack of awareness of them. Despite the dominance of urbanism in the modern world we still lack a sociological definition of the city which would take adequate account of the fact that while the city is the characteristic locus of urbanism, the urban mode of life is not confined to cities. For sociological purpose a city is a relatively large, dense, and permanent settlement of heterogenous individuals. Large numbers account for inidividual variability, the relative absence of intimate personal acquaintanceship, the segmentalization of human relations which are largely anonymous, superficial, and transitory, and associated characteristic. Density involves diversification and specialization, the coincidence of social relations, glaring contrasts, a complex pattern of segregation, the predominance of formal social control, and accentuated friction, among other phenomena. Heterogeneity tends to break down rigid social structures and to produce increased mobility, instability, and insecurity, and the affilitation of the individuals with a variety of intersecting and tangential social groups with a high rate of membership turnover. The pecuniary nexus tends to displace personal relations, institutions tend to cater to mass rather than to individual requirements. The individual thus becomes effective only as he acts through organized groups. The complicated phenomena of urbanism may acquire unity and coherence if the sociological analysis proceeds in the light of such a body of theory. The empirical evidence concerning the ecology, the social organization, and the social psychology of the urban mode of life confirms the fruitfulness of this approach.