Boundaries are not only pre-conditions of politics, but also indispensable elements of political and social thinking. Borders, boundaries and limits – understood in a broad sense – are omnipresent, shape political and social realities, and are part of our daily experiences. Nonetheless, in Political Theory as well as in International Relations Theory until recently there was not much interest in discussing boundaries in a systematic and comprehensive manner. “Boundaries creep into political philosophy without us noticing.” – This statement of Onora O’Neill still seems to be true. One reason for this rather contained interest might be the negative interpretation and connotation of boundaries. Already by pure intuition borders, boundaries and limits seem to be disruptive, separative, inhibitive and prohibitive instances. At the same time, the structures, aims and functions of boundaries – as well as the role of “deboundarizations” – become increasingly important in an era of denationalization, transnationalization and globalization. It is therefore important to take a close and systematic look at borders, boundaries and limits.