The concept of the public realm is the most fundamental of all political concepts because it is only the shared relationship it constitutes between rulers and ruled that makes government more than mere domination. It is therefore not surprising that the question of how the public realm is to be defined has been a central concern of political thinkers from Plato to more recent philosophers like Hannah Arendt. Although the answers they have given have of course varied greatly, what is relevant in the present context is simply the assumption, for well over two thousand years, that there could be an answer. What is novel today, by contrast, is the increasing realization that political theory cannot in principle provide philosophical guidance of any kind about how the public realm can be finally and definitively identified, in a way which would clearly distinguish it from the private realm and thereby enable an incontestable limit to be set to the proper scope of politics and of state action. The present essay considers how some leading contemporary political thinkers have responded to this situation.