This study intends to follow the semantic evolution of the concept of fides in Cicero’s thought, from the rhetorical and oratorical works to the political ones till to the philosophical treatises of the latest years. One can generally say that before 54 b.C. , the date of publication of the De re publica, Cicero, a great user of the concept, at least in the rhetorical field, has a rather confused notion of it, founded on the search much less for its precise sense than for the effectiveness of its use. From this treaty he begins to wonder about its origin: if fides, as he believes, has a natural origin, how to explain its detachment from the world’s practices? It is, therefore, to understand what fides’ naturalness means; and this is what Cicero tries to do in the philosophical works of the latest period. First of all in the Academica he investigates the place fides occupies at the level of knowledge, in opposition to Lucullus’ dogmatic perspective. At the end of this reflection, permeating also the De Finibus and the De Natura Deorum, it comes out confirmed the natural origin of fides, on the condition that you rescue the concept of nature from the Epicurean reconstruction of axiology, founded on sensation and voluptas. This is what Cicero devotes himself to in his very last work, the De officiis, seeking to make of fides the very foundation of iustitia but also trying to reconcile its roots in nature with the variability of the circumstances: that is, giving a less intransigent interpretation of it, while maintaining its ideal value, which finds embodiment in the figure of Regulus.