This chapter examines how European thinkers working from within and without the Frankfurt School of critical theory have understood the public sphere as a distinctive political category. First-generation members of the school rejected institutional democracy and mass politics as ideologies that mask domination. The succeeding generation, whose most important representative is Jürgen Habermas, rejected that diagnosis. Habermas’s more optimistic assessment of the emancipatory potential of the public sphere as a medium of rational learning sought a middle ground between critics and defenders of liberal democracy. This ambivalence provoked strong counter-reactions from systems theorists, such as Niklas Luhmann, and from adherents of theories of agonal democracy descended from Carl Schmitt, on the right, and Hannah Arendt, on the Left. As we shall see, these reactions are amplified by those who seek to extend the public sphere beyond the boundaries of the nation state.