Prevention of environmental harm is commonly regarded as an obligation established by a customary rule of international law. However, many questions are still unanswered, including the exact content of such an obligation. This book attempts to address those questions by proposing a different reconstruction: it is argued that prevention of environmental harm amounts to a general principle of international law, rather than a customary rule. To this end, the book discusses the weaknesses of the classifìcation of prevention as a customary obligation. It then demonstrates that prevention of environmental harm has actually operated as a general principle, inspiring the formation of more specifìc rules and performing a guiding function within international case law. In light of this, the problem of the content of prevention under general international law is reformulated as one of identifying the customary rules embodying it. Since the dilution of prevention of environmental harm by the concept of sustainable development is apparent from the current trends within customary international law, the book concludes by exploring what role the principle of prevention might play in the future, particularly in a de-growth scenario.