This introduction sets out the development of International Relations (IR) theory and its applicability to the G20 and global governance. The chapter contends that IR theory is a field of scholarship which aspires to consider and guide practical action in respect to world politics. IR theory is therefore best seen as a ‘practical discourse’ involving specific types of systematic assumptions about how the world works. In doing so, this body of theory considers broader questions of power, a wider range of political problems and limits as well as interrogating the ethical issues involved in world politics. This introduction to the volume sets out what IR theory is, what the main elements of the declared purpose and operation of the G20 are and broadly articulates why considering IR theory is important to the future of the G20.