The paper reconstructs Carnap's epistemological and ontological ideas stressing the link between these ideas and the most general tenets of Logical Empiricism (negation of the Kantian theory of synthetic a priori judgments, linguistic theory of the a priori, influence of Poincaré's conventionalism, principle of verification, refusal of metaphysical absolutism). From this point of view it also discusses both the Carnap/Quine debate on analyticity and ontology and the difference between Carnap and the "young' Reichenbach on the nature of the relativised a priori.