In this book Jonathan Sperber deploys his extensive knowledge of nineteenth-century European social and political history, and his diligent research into sources that have become readily available only recently, to produce a substantial biography of Karl Marx. We find, however, that Sperber is mistaken in his treatment of Marx's ideas and of the intellectual contexts within which Marx worked. In fact, we suggest that he is systematically mistaken in this regard. We locate a root source of the error in his reductive approach to theoretical ideas. In section I we focus on the claim, taken for granted in the book, that Marx's ideas are instantiations of “materialism.” By detailed reference to the record of Marx's writings, we show that there is no justification for describing Marx as a “materialist” in the usually accepted senses of that term. In section II we review how Soviet and other interpreters of Marx, taking their lead from the later Engels, insisted that “materialism” was fundamental to Marxism. We suggest that Sperber's presentation of Marx's thinking as “materialist and atheist” aligns far better with such interpretations than it does with what Marx actually wrote. In sections III and IV we criticize Sperber's “contextualist” approach to dealing with ideas in history. His approach may seem reminiscent of Quentin Skinner’ s, but where Skinner deploys the discursive conventions prevailing in a past time to illuminate theoretical ideas, Sperber reduces theoretical ideas to context. We name Sperber's approach “theoretical nominalism,” a term that we use to denote the view that theoretical ideas are nothing but interventions into particular situations. We end by suggesting that greater attentiveness to philosophy and theory would have enriched Sperber's efforts in this book.