Foundational norms and ideas that guide the development, interpretation, and application of environmental law, and encompasses a wide variety of principles, with some being incorporated in treaty texts and others gaining customary status. Principles of international environmental law are an important building block and their usage is widespread. Some are included in non-binding instruments, including political instruments, while others are enshrined in issue-specific multilateral environmental agreements that are legally binding. When enshrined in such agreements, the scope of the principles is confined to that particular agreement. However, those principles that are not contained in multilateral environmental agreements also play an important role in guiding the interpretation and further development of those agreements. More generally, principles also serve to supplement or complement more specific rules (gap-filling function). The general character of the principles permits their application to the continuously evolving interrelationships between human activity and the environment. An analysis of the general principles and rules of international environmental law is found in Sands et al., Principles of International Environmental Law (4th edn) Cambridge University Press, 2018, 197-251.