Concluding that the precautionary principle embodies customary international law is one thing. Determining what this means is quite another. That challenge is met by this work, which resolves a number of crucial questions concerning the scope of this principle of international environmental law; the conditions triggering a right or duty to take precautionary action; the measures to be taken; the allocation of the burden of proof; and the role of socio-economic factors. These questions are dealt with one at a time through the charting and analysis of patterns and common denominators in the extensive (inter)national practice of states regarding the precautionary principle. The hard legal core of the principle is thus gradually exposed. In the process, a realistic and accessible account is given of how and to what extent this general principle can and does direct the actions of states in concrete instances. Ultimately, this work sets out what it takes to act in conformity with the precautionary principle under general international law, and will be of interest to anyone involved with international law and environmental protection.