The content and status of the precautionary principle remains highly debated and various questions arise, such as its status as a rule of customary international law, including its scope, addressee, triggering threshold, precautionary action measures, and eventually limits of the principle. Thus, this book examines the present state of affairs regarding the implementation of the principle in the law of the sea in different sectors, e.g. pollution of the marine environment, conservation and management of living marine resources, and transboundary transports of radioactive and hazardous wastes. In addition, it extracts evidence of its acceptance as part of customary international law, and indicates that below this level there is also an emerging practice of international law of applying the precautionary principle in a common way.