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This chapter describes the concept of precaution in international environmental law, which concerns anticipatory action in response to scientifically uncertain threats of environmental harm. Its most frequently referenced formulation can be found in Principle 15 of the Rio Declaration on Environment and Development. The Rio Declaration's endorsement of precaution in Principle 15 introduced to international environmental law a new discourse over the appropriate evidentiary foundations of global environmental regulation. The chapter then focuses on four key questions (and attendant debates) regarding precaution that have been critical in understanding its role in international environmental law. These questions concern the meaning of precaution as a conceptual pillar of international environmental law; the legal status of precaution as a principle of international environmental law; the formulation and understanding of precaution evident from international environmental treaties and case law; and the consequences of applying precaution in decision-making concerning threats of environmental damage.

This chapter provides an overview of international environmental law, which is the legal and regulatory framework devised by the community of sovereign states to address global environmental problems. These problems include the potential for runaway climate change, vanishing biodiversity, increasing freshwater scarcity, and severe degradation of marine resources and ocean ecosystems. In the last decade, international environmental law has acquired further breadth, depth, nuance, complexity, and reach. In particular, it is more deeply interconnected with policy and legal efforts in many other fields, including international trade and investment, human rights and migration, energy, disaster response, armed conflict, technology innovation and intellectual property protection. The chapter then explores and illustrates the ways in which international environmental law has evolved over the last decade. It highlights how the field has adapted to a changing geo-political context, as well as to the possibilities and limits of global regulation in addressing the complex, polycentric, and intractable nature of environmental harms.