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-running constructivism-realism debate can be considered as an acknowledgement on both sides that objects are real (i.e. pre-existing the situation) and constructed at the same time. The second thematic complex focuses on the role of instruments and classification in establishing not only relations of heterogeneity, but also of stability. The third thematic complex broadens the debate and actively seeks to promote an STS-driven ontological turn for research concerned with the humanities and the social sciences more generally. This study is based on both quantitative and qualitative interpretations of the literature.