This chapter considers the notion of a formal ontology, which is a conceptual vocabulary equipped with a logical semantics. Three families of knowledge representation and reasoning formalisms that put ontologies at the core of any knowledge base are presented, namely: description logics, conceptual graphs and existen-tial rules. We present the main knowledge constructs and dialects of these families, as well as the main reasoning problems with their complexity. We highlight the relationships between these families and compare them from an expressivity viewpoint.