The aim of this paper is to show the originality of Rousseau’s conception of modesty (pudeur) when compared to the eighteenth-Century debate, in which two sharply contrasting explanations for this feeling were opposed. Some thinkers considered modesty as a natural and original inclination, while others as a conventional product of social institutions. On a superficial reading of his work (especially as far as the Lettre à d’Alembert is concerned), Rousseau seems clearly to embrace the first hypothesis; nevertheless, a deeper analysis of his conception reveals its complexity. For Rousseau, modesty cannot be trivially traced back to a “first” (original but pre-moral) naturalness; rather, it must be brought back to a “second” naturalness, which arises from the dynamics of the social “supplement.” In this perspective, modesty can be good (moral and social) or bad (moral and conventional); for this reason the study of its bon usage will be placed at the center of Rousseau’s moral reflection on woman.