The noun “radical” as applied to reformers who held advanced views came into general use in the early nineteenth century. It was at first used in a derogatory sense, denoting, as Walter Scott wrote in 1819, “a set of blackguards.” However, it was taken up by the subjects of the intended abuse and quickly acquired a certain respectability—so much so that by 1830 one middle-class radical was recording that “the term Radical once employed as a name of low reproach, has found its way into high places, and is gone forth as the title of a class who glory in their designation.” Reformers from across the political spectrum were soon being designated “radical,” as can be seen from the application of the term to individuals as diverse in outlook as Lord Durham, Richard Oastler, and Bronterre O'Brien.This eclecticism has led historians to pronounce the concept useless as a tool of historical analysis. If any tradition at all emerges from studies of English radicalism, then it is a tradition of liberal humanitarianism, a pattern of reform that is nonclass and nonideological. At best “radical” retains its original adjectival power of describing a root-and-branch reformer, an individual who worked to change quite substantially existing economic, political, or social structures by word or deed. But the evident contradictions and discontinuities in the so-called “radical tradition” have made historians balk at making further claims than these. This essay, however, is a provisional attempt to shift the focus away from the personalities and the specificities of reform movements in their peculiar conjunctural moments to the operation of radicalism as a powerful ideology that, far from being nonclass and nonideological and despite (or even possibly because of) its internal contradictions, has profoundly influenced class development and class relations.