This essay analyzes the subject of ontology starting from the categorical issues, with special reference to the reflection pursued within the scope of Aristotelian tradition. After ontology has been distinguished both from metaphysics and gnoseology, the problem of ontological commitment and of individuation principle has been addressed in the attempt to provide a positive characterization of ontology, starting from the issue of existence and the modality to be of being. Through an investigation concerning the debate on so-called universals, the layout of fundamental Heideggerian ontology and the distinction between descriptive terms and transcendental terms, the essay comes to mention some problematic passages allowing to characterize the ontology in the encounter between reality and language, showing how in the sign of pluralistic realism of Aristotle the ontology stabilizes of necessity on the level of contingency.