Samuel Huntington's analysis of the “Hispanic challenge”—his claim that Mexicans are on their way to forming a separate nation within the U.S.—rests on a series of misconceptions that are not his alone. At the heart of his difficulties is a widely shared form of reasoning about racial and ethnic populations that has become increasingly problematic in the contemporary era of mass immigration: it anticipates a single, predominant outcome for group members, such as assimilation or racialized exclusion; instead, it is the diversity within groups of patterns of incorporation into American society that needs recognition today. This is all the more true of Mexican Americans because of the long history across which their immigration stretches and their presence in the Southwest and California before the arrival of European Americans.Richard Alba is Distinguished Professor of Sociology and Public Policy at The University at Albany, SUNY ([email protected]). This paper was written while the author was a Fellow at the Radcliffe Institute for Advanced Study at Harvard University. He is grateful to Jennifer Hochschild for the invitation to write it for this symposium and to his colleagues at the Institute, especially Susan Eckstein and Luis Fraga, for their helpful comments on earlier drafts.