If the basic problem of the international is societal multiplicity – the simple but surprisingly consequential fact that the world is divided into distinct societies (Rosenberg 2006) – the basic problem of the global environment could be said to be planetary singularity; that we all inhabit, in the end, one finite interconnected space. Together these two starting points make for the basic conundrum of Inter- national Relations and the Earth: how does a divided world live on a single globe? This introduction first provides an overview of the recent rise of ‘the environment’ in international politics and offers an account of how this builds on older ways in which the natural world has made up part of the stuff of international politics. Second, it surveys the main traditions and approaches to studying International Relations of the environment, painting a picture of diversification in two senses: from the study of ‘environmental multilateralism’ towards a broader ‘global environmental politics’, and from ‘problem-solving’ to a greater diversity of ‘critical’ approaches, some of which originate in disciplines outside core IR territory. While the traditional problem-solving approaches have tended to treat the environment as just another issue for International-Relations-as-usual, critical approaches have begun reflecting on the theoretical implications of taking environmentalism seriously (see also Eckersley 2013). Third, the direction of inquiry is therefore reversed to ask, in effect, ‘what has the environment ever done for IR?’, before the plan for the rest of the book sketches the content and direction of the ensuing chapters that explore the problematique of International Relations and the Earth.