The term ‘ontology’ refers to a philosophical discipline which can be defined in relation to its object (e.g. ‘being’, or ‘entity’) or in relation to other philosophical disciplines such as e.g. metaphysics or epistemology. As the science of ‘Being’, ontology is concerned with the most general and fundamental properties, modes, and aspects of being, or of entities in general, as illustrated in the work of Husserl and Heidegger. Although this essay will be more concerned with what is usually called ‘formal’ ontology, which combines the methods of ‘informal’ (classical) ontology with insights from modern formal methods in symbolic logic and aims at the development of a logic of all forms and modes of being, it will use traditional ontological views to clarify certain points of view.