||Following Quine, ontology is here understood as the study of what there is. (Re: neo-Aristotelian ontology, the study of what grounds what, see the "Fundamentality" category.) Our focus is on the existence of the most generic things that populate many philosophers' ontologies, e.g., objects, properties, natural kinds, states-of-affairs, events, etc. We often talk of these things without thinking twice, but the existence of such entities can seem odd on reflection. For instance, it is natural to say that red roses and red firetrucks have something in common, the property of being red. But does this mean there is a single entity that is a constituent of *every* such rose and firetruck? A second example concerns composite objects: Suppose Abe Lincoln replaces the handle of his axe in 1825, and later in 1860 replaces the head. Does this mean he has owned more than one axe in his lifetime? In general, given a puzzling entity X, Realists about X will strive to minimize such oddities--whereas Anti-Realists often try to preserve ordinary talk of X, despite excluding X from their ontology. Questions about ontology can also lead to questions about these questions. Thus, ontology often bleeds into metaontology, the study of the study of what there is. In recent years, the ontology literature has grown dramatically, especially on metaontology and on composition.