This article places the growth of an urban underclass in the broader context of trends in inequality and the stratification of place in global cities. Using Chicago as a case study, we construct a multidimensional typology of urban neighborhoods to illuminate trends in the spatial distribution of opportunity, the impact of immigration on the city's ecological structure, and modal patterns of neighborhood change. Our empirical analysis documents (1) the increasing spatial polarization of Chicago's neighborhoods, fueled by a concentration of both affluence and socioeconomic disadvantage; (2) the erosion of working-class areas as an ecological category; (3) the emergence of Hispanic neighborhoods as a distinct ecological type; and (4) the existence of race-specific patterns of neighborhood upgrading. These trends have broader implications for the study of spatial inequality in advanced industrial cities.