The study of foreign policy is usually concerned with the interaction of states, and thus with governance structures which emerged either with the so-called 'Westphalian system' or in the course of the 18th century: diplomacy and international law. As a result, examining foreign policy in earlier periods involves conceptual and terminological difficulties, which echo current debates on 'post-national' foreign policy actors like the European Union or global cities. This volume argues that a novel understanding of what constitutes foreign policy may offer a way out of this problem. It considers foreign policy as the outcome of processes that make some boundaries different from others, and set those that separate communities in an internal space apart from those that mark foreignness. The creation of such boundaries, which can be observed at all times, designates specific actors - which can be, but do not have to be, 'states' - as capable of engaging in foreign policy. As such boundaries are likely to be contested, they are unlikely to provide either a single or a simple distinction between 'insides' and 'outsides'. In this view, multiple layers of foreign-policy actors with different characteristics appear less as a modern development and more as a perennial aspect of foreign policy. In a broad perspective stretching from early Greek polities to present-day global cities, the volume offers a theoretical and empirical presentation of this concept by political scientists, jurists, and historians.