Substance has long been one of the key categories in metaphysics. This Element focuses on contemporary work on substance, and in particular on contemporary substance ontologies, metaphysical systems in which substance is one of the fundamental categories and individual substances are among the basic building blocks of reality. The topics discussed include the different metaphysical roles which substances have been tasked with playing; different critieria of substancehood (accounts of what is it to be a substance); arguments for and against the existence of substances; and different accounts of which entities, if any, count as substances.