The controversial question of whether or not at present the precautionary principle is to be considered a norm of customary international law is the key theme of this work, which treats the issue as part of a broader discussion of the principle's legal status on the international plane. This discussion, in turn, is put in perspective by an account of the history of the principle in international environmental law and policy.;The greater part of this study consists of the mapping and analysis of state practice in respect of the precautionary principle. The book then applies the generally accepted principles governing the formation of customary international law to this body of state practice. Relevant international judicial decisions and the opinions of scholars are treated as well.