Formal ontology as a main branch of metaphysics investigates categories of being. In the formal ontological approach to metaphysics, these ontological categories are analysed by ontological forms. This analysis, which the Element illustrates by some category systems, provides a tool to assess the clarity, exactness and intelligibility of different category systems or formal ontologies. It discusses critically different accounts of ontological form in the literature. Of ontological form, the authors propose a character-neutral relational account. In this metatheory, ontological forms of entities are their standings in internal relations whose holding is neutral on the character of their relata. These relations are 'formal ontological relations'. The Element concludes by showing that our metatheory is useful for understanding categorial fundamentality/non-fundamentality, different formal ontologies, and for unifying metaphysical questions. This title is also available as Open Access on Cambridge Core.