The contribution aims to analyze Cicero’s overall thought regarding nature as a ‘source’ of law. First, the passages from his rhetorical works are examined, where he is concerned with enumerating and defining the individual ‘parts’ that make up the Roman legal system; the attempt is then made to place this thought, in its broader philosophical scope, within the complex cultural and political context in which Cicero was immersed. In his more mature writings, from De Republica to De Officiis, the natural law tòpos takes on a central role in the political and intellectual struggle that he waged, until his very last day, in defense of the Republic – or at least of a certain conception of it.