The purpose of this article is to contribute to the clarification of the legal scope of the precautionary principle. The primary focus is on general international law, so as to determine as accurately as possible what it is that ‘the’ precautionary principle entails. The legal development, status and implications of the principle pass in review. A definition is presented of a right and a duty of States to take precautionary action, which is deemed representative of the current state of customary international law. Several particularly controversial issues receive separate treatment, including the burden of proof, the role of uncertainty and the application of the precautionary principle to human health protection. Special attention is paid to the practical relevance of the various findings, which are placed in the context of recent developments.