Marx and Engels’ concept of the “lumpenproletariat,” or underclass (an anglicized, politically neutral term), appears in The Communist Manifesto and other writings. It refers to “the dangerous class, the social scum, that passively rotting mass thrown off by the lowest layers of old society,” whose lowly status made its residents potential tools of the capitalists against the working class. Surprisingly, no one has made a substantial study of the lumpenproletariat in Marxist thought until now. Clyde Barrow argues that recent discussions about the downward spiral of the American white working class (“its main problem is that it is not working”) have reactivated the concept of the lumpenproletariat, despite long held belief that it is a term so ill-defined as not to be theoretical. Using techniques from etymology, lexicology, and translation, Barrow brings analytical coherence to the concept of the lumpenproletariat, revealing it to be an inherent component of Marx and Engels’ analysis of the historical origins of capitalism. However, a proletariat that is destined to decay into an underclass may pose insurmountable obstacles to a theory of revolutionary agency in post-industrial capitalism. Barrow thus updates historical discussions of the lumpenproletariat in the context of contemporary American politics and suggests that all post-industrial capitalist societies now confront the choice between communism and dystopia.