In Germany, the concept of "co-" is always present: cohabitant, colleague, fellow human, but above all politically: fellow citizen. The same applies to "integration." "Foreigners" must be integrated, the Federal President has an "integration function," and even the Constitutional Court is expected to "integrate." This sense of community is so powerful in the political culture that a whole theory of constitutional law emerged from it in Weimar: the integration theory of Rudolf Smend. In the Federal Republic, it became an "official" state doctrine, even though its German nationalist author originally positioned it against liberal democracy. This volume brings together contributions critiquing Rudolf Smend's theory in the context of the political theories of Hans Kelsen and Carl Schmitt, as well as its reception in the understanding of constitutional bodies.