This text is a systematic treatise on formal ontology or general metaphysics. Ontological issues - states of affairs, properties, actual existence, identity, exemplification, and others - are always discussed within, and in relation to, formal axiomatic systems. Starting with a theory of states of affairs, these systems become more and more comprehensive, complex and powerful, until finally the level of a universal ontology of intensional entities is reached. The central unifying notion throughout the constructions is that of part. There are many occasions on which historical ontological theories are discussed, notably those of Leibniz. The book should appeal to ontologists and metaphysicians who have a training in formal logic and who are interested in systematic ontological theories which are logically precise both with respect to the deduction of principles and the definition of concepts. It should also be of interest to analytic philosophers and historians of philosophy.