This was a lecture delivered on November 14th, 1942, as one of a series of seven open Lectures on Eminent Economists arranged by the Faculty of Economics and Politics at Cambridge during the Michael-mas Term of that year. In the latter part of section 2 some paragraphs have been included which are extensions of the original lecture; these paragraphs being taken from a pamphlet by the present writer, entitled Marx as an Economist, an essay (Lawrence and Wishart, London, 1943), by kind permission of the publishers. For most of its biographical data, especially in the early life of Marx, this lecture has drawn heavily upon the standard biography of Marx by Franz Mehring, Karl Marx: the Story of his Life (trans. Edward Fitzgerald, London, 1936.)