This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book seeks to explain comparatively why and how sub-state governments (SSGs) conduct their international relations, and how federal authorities and local governments coordinate in the definition and implementation of national foreign policy. It aims to analyze why and how sub-state governments conduct their external affairs with international actors, the causes and consequences of paradiplomacy and how federal state and sub-state governments coordinate in the creation and implementation of foreign policy. Paradiplomacy, or the international relations of sub-state governments, is a worldwide phenomenon. Non-state actors such as multinational corporations, non-governmental organizations and diverse interest groups have also increased their participation in external affairs; this has been called citizen or social diplomacy. Sub-state governments are members of international organizations and agencies; there are more than 125 multilateral arrangements of SSGs.