The ontological turn in social theory is a variegated phenomenon. Ontological turn is not the only expression used to identify it. Others include material turn, co-production, continental materialism and realism, new materialism. Marx and Marxism constitute the most conspicuous tradition of materialist outlooks on the relationship between nature and society. This tradition, however, does not offer a univocal picture. In particular, post-structural Marxism, as emerged between the 1960s and 1970s, sought to disentangle materialism from any trace of idealism and teleological evolutionism. Louis Althusser is key to this endeavour. For Althusser, idealism is a philosophy of origins and ends; an onto-teleology that builds on the alleged essential character of the world and its eventual destiny. For post-structural Marxism the engine of nature's historical dynamics remains human action. Actor-network theory (ANT) case for a flat ontology looks also less radical than it may seem when one reflects on how it pivots on a neat distinction between human and nonhuman entities.