This Element gives an introduction to the emerging discipline of natural language ontology. This is an area at the interface of semantics, metaphysics, and philosophy of language that is concerned with which kinds of objects are assumed by our best semantic theories. The Element reviews different strategies for identifying a language's ontological commitments. It observes that, while languages share a large number of their ontological commitments (such as to individuals, properties, events, and kinds), they differ in other commitments (for example, to degrees). The Element closes by relating different language- and theory-specific ontologies, and by pointing out the merits and challenges of identifying inter-category relations within a single ontology.