The chapter is organized into three main sections. The first section (paragraphs 1–4) presents basic preliminary distinctions, such as those (1) between ontology and metaphysics, (2) among descriptive, formal and formalized ontologies, (3) between pure ontology and its presentations, and (4) between ontology and epistemology. The second section (paragraphs (5–7) is substantive. After defending the idea of ontology as the theory of the most general categories, and presenting the difference between substance and its determinations, the chapter claims that proper understanding of substance requires at least six different theories, five directly dealing with its internal configuration and one dealing with both internal and external aspects. These six ontological sub-theories address the following problems: (1) the differences among such general categories as object, process, event, state of affairs, stuff, group etc.; (2) classification; (3) structure, or types of whole and their parts; (4) chronotopoids, or types of spaces and times; (5) interactions, or forms of causation; and (6) levels of reality, or the distinction among the basic regions of reality, such as the material, the psychological and the social ones. The third section (paragraphs 8–11) touches briefly upon some of the remaining issues, for instance determinations, the substance-determination relation and predication and non-commutative categorical frameworks.