This work makes clear what it means that the precautionary principle represents customary international law Through the analysis of state practice regarding this principle of international environmental law it answers crucial questions concerning the conditions triggering a right or duty to take precautionary action the nature and content of such action the issue areas to which the principle applies the allocation of the burden of proof and the role of socio economic factors Ultimately it details what it takes to act in conformity with the precautionary principle under general international law