Modern public environmental law is a field largely premised on prevention. From the 1970s onward, international and domestic environmental law has been shaped by the recognition that strictly remedial approaches to environmental issues are not adequate for deterring environmental harm and stemming serious environmental degradation. 1 The United Nations General Assembly 1992 Rio Declaration on Environment and Development (Rio Declaration) helped to solidify four guiding principles for public environmental law: polluter pays, prevention, sustainable development and precaution. 2 Of these four principles, only polluter pays – the idea that she or he who causes the pollution should bear its costs – is retributive. Prevention is the idea that we should take steps to prevent or mitigate foreseeable harm to the environment. It is often implemented through the requirement of conducting an environmental impact assessment prior to undertaking any regulatory activity. Sustainable development, an emerging and complex body of international law, has a significant preventive component. 3 It captures the idea 24that the present generation has an obligation to future generations not to push the Earth’s ecological capabilities to their limits. The fourth principle – and the focus of this chapter – is the precautionary principle, 4 which posits that a lack of scientific certainty should not pose a barrier to state action in the face of serious or irreversible threats to the environment.