Introduction
The introductory chapter presents the relevance of the topic in the framework of the practice of international organizations and existing legal scholarship. In particular, it describes how scholars and practitioners do not share a common understanding of what an international organization is and the consequences of this absence of agreement. The main claim is that in order to conceptualize international organizations we have to look at the characteristics of the legal systems they develop and the legal nature of their rules. Four main theses are presented: functionalism (international nature), constitutionalism (internal nature), exceptionalism (only some organizations develop internal rules), and informalism (only some rules have an internal nature). Finally, it sets up the aim of the book: to analyse different conceptualizations, to assess the existence of a general regulatory framework, and to provide a definition of the concept of an international organization in international law.