This review assesses sociological and historical research relevant to the emergence and consolidation of the American middle class in the nineteenth century. On the one hand, macrosociological theories have relied on a two-class model which renders the middle class a residual social category. Yet, on the other hand, community studies of the “new” social history—while they have opened up new avenues of inquiry into the complex social processes underlying middle class formation—have tended to focus on particular decades of the nineteenth century, leading to a fragmented view of the occupational composition of the middle class. Distinct literatures have developed around the study of particular occupational strata: artisans, small capitalists, white-collar wage earners, and the petite bourgeoisie. We argue here that different occupational groups overlap in time and represent a heterogeneous and historically shifting middle class rather than distinct entities. The argument for the integrity of a distinct middle class also rests on an understanding of the development of urban institutions and the cultural expressions of middle class lifestyles and behavior. The expansion of this middle class, however, was closely linked to a growing economy and increasing equality of opportunity. We speculate that the reversal of these conditions, evident from the 1970s, may undermine the well-being of the middle class and its correlative social values, notably tolerance and civility.