This paper discusses the significance of the concept of a social formation for historical materialism. It argues that the concept is wrongly thought to be associated uniquely with the writings of Louis Althusser and with structuralist Marxism. It can be found in the writings of Marx himself, as well as those of Lenin, and is central to an adequate understanding of classical Marxism. To illustrate its importance the paper shows how the concept may be used to shed new light on (...) the debate around the transition from feudalism to capitalism. (shrink)