This work investigates Foucault s relationship with the polemical thematic of post-modernity through the Foucault-Habermas debate. Postmodernity is a controversial concept, that encamps numerous readings and characterizations. We chose to dialogue with Habermas, not only because of the German philosopher s importance in the crystallization of this concept throughout the 80 s, but also because he criticize Foucault s philosophy directly. Habermas understands modernity as an unfinished project, yet to be rescued and completed by the development of a communicative rationality. His theory of modernity, then, aims to combat emerging postmodernity, marked by irrationalism and relativism. Habermas sees in Nietzsche the inflection point for postmodernity. The nietzschean genealogy, which will also be that of Foucault s, appears to Habermas as a kind of devastating critique of reason, that takes place on the outer limits of modernity: they both unmasks reason as a mere will to power and domination. We then try to show how the French philosopher dealt with the theme of modernity. For Foucault, if modernity is also characterized by the Enlightenment, it is much more for the critical attitude that it evokes, than for a certain model of rationality considered as ideal. Foucault argues that the Enlightenment opened the philosophical reflection for the investigation of who we are in our present, of what is our present, in a way that consolidated the effort to situate ourselves critically against the rationalities that guide us and that govern us. Foucault then emphasizes the kantian notion of adulthood: being modern is very simply the refusal to be tutored. In this sense, the whole work of Foucault complicates the classical Illuminist narrative, which is usually presented in terms of a progress of the Lights, a struggle of reason against the shadows, knowledge against ignorance and prejudices, etc., by showing that there are authoritarian elements inside the Enlightenment movement. The Enlightenment not only represents the movement of questioning the authority of the Church - it represents the questioning of all forms of authority. If modern reason becomes a new source of authority criticism must radicalize itself into a metacritical posture in the order to apprehend the new rationalities that govern us today.