In the Westphalian system States had been at center of international legal system and the international law had been only concerned with the rights and duties of States until mid-20th century. Wide proliferation of activities of non-State actors in the international arena in recent years makes us to rethink the exclusive role of States in the traditional areas of international law. This article, as a kind of prologue at the seminar on“Non-State Actors in International Law” organized by the Seoul International Law Academy, raises some basic questions: Who are non-State actors?, Why do we need to talk about them?, What is legal implication of the activities of non-State actors in international forum?, How do we have to translate non-State actors into Korean? This article does not necessarily try to provide clear answers to these questions, but asks more debate among academic circle about this topic.