We examine the evidence for the claim of an ‘ontological turn’ in science and technology studies (STS). Despite an increase in references to ‘ontology’ in STS since 1989, we show that there has not so much been an ontological turn as multiple discussions deploying the language of ontology, consisting of many small movements that have changed the landscape within STS and beyond. These movements do not point to a shared STS-wide understanding of ontology, although it can be seen that they do open up STS to neighbouring disciplines. Three main thematic complexes are identified in this literature: constructivism and realism; instruments and classification; and the social sciences and the humanities. The introduction of ontology into the long-ru...