This chapter examines Karl Marx's relationship to Friedrich Engels and their joint works of the 1840s, along with those works each of them published separately. Marx is regarded as Engels regarded him; that is, as the more important of the two, both as a theorist and political activist in the First International. The chapter begins with a discussion of the Manifesto of the Communist Party, with particular emphasis on Marx and Engels's views on ideology. It then considers Marx's critique of political economy; his concepts of use value, exchange value, and surplus value; and the ‘fetishism of commodities’ as discussed in the first volume of Capital. It also explores Marx's insights about Western European history and his theory of the state before concluding with an overview of Engels's contribution to Marxism.