The "underclass" concept pervades social science research on poverty, racial relations, and more recently, immigration. In this article, we elaborate and extend Massey's critique of the underclass concept by briefly reviewing the history of this concept and emphasizing its contemporary application to immigrants and undocumented workers. We also explore how the term as well as popular variants, including the "rainbow underclass," are ambiguously defined yet legitimated by contemporary researchers and, as a result, have become more deeply embedded in social science scholarship. Furthermore, we critique the prevalent use of this term in describing minority groups and highlight its potential to reaffirm individualistic, racist views of immigrants while obscuring social processes that perpetuate inequality. We conclude with a call to scholars and policy analysts to replace this term with pointed analyses of the structural conditions that shape the lives of disadvantaged groups. (Contains 4 notes.)