 Ontological dependence is a relation—or, more accurately, a family of relations—between entities or beings (onta in Greek, whence ontological). For there are various ways in which one being may be said to depend upon one or more other beings, in a sense of “depend” that is distinctly metaphysical in character and that may be contrasted, thus, with various causal senses of this word. More specifically, a being may be said to depend, in such a sense, upon one or more other beings for its existence or for its identity. A traditional approach was to analyze all forms of ontological dependence in modal terms, that is, in terms of distinctly metaphysical notions of possibility and necessity, but more recently an analysis in terms of the notion of essence has become popular. A third option is to treat the notion of ontological dependence as a metaphysical primitive, in which case it is not possible to give a further definition of it. This entry will remain neutral about the ultimate basis of ontological dependence. At least in some cases, it is possible to use ontological dependence as a tool to describe the relationship between entities without taking a stand about its basis, but in other cases this may not be possible. Notions of ontological dependence are frequently called upon by metaphysicians in their proposed analyses of other metaphysically important notions, such as the notion of substance or the thought that tropes depend for their existence on a substance.