This chapter considers the relationship between scientific and philosophical approaches to ontology, with the aim of clarifying what it means to engage in the project of scientific ontology. It introduces the most influential conceptions of ontology to emerge in the history of philosophy of science. These include deflationary views, which redescribe talk of ontology in terms of other things, as well as views which, conversely, take ontology at face value as an inquiry seeking knowledge of what there is in the world—a world whose existence is independent of the thoughts one may have concerning it. It is argued that the sciences do not yield ontologies until and unless they are interpreted, which requires some recourse to philosophical thinking, and that case studies of science cannot by themselves settle disputes about how these interpretations should go.