Increasingly, poverty has become concentrated among the urban underclass in the United States. The dramatic economic changes underlying this situation have taken place within the context of larger changes in global, national, and regional political economies, including such factors as industrial restructuring and migration flows. In addition, the size and characteristics of an urban underclass are defined by the concepts of concentrated and persistent poverty and of "underclass behaviors." Although analysts have provided estimates of the size of this population, no consensus exists concerning the causes and dynamics of the problem. This paper surveys research on the urban underclass and describes a new program of the Social Science Research Council encouraging investigation of the structures and processes which generate, maintain, and can overcome urban poverty. The following levels of analysis are discussed: (1) changing global and national economies, including employment trends and shifts in the nature of labor; (2) regions, states, and cities; (3) communities and neighborhoods; and (4) family structure and processes. Several mechanisms comprise the nucleus of discussion and research: (1) structures that produce or create the possibility of urban poverty; (2) precipitating events that cause individual or family impoverishment; and (3) cycles or dynamics that maintain or enable one to break out of persistent poverty. A selected bibliography of 43 items is appended. (AF)