International law governing marine environmental protection is not merely a mosaic of specific rules; rather it must be considered as a system governing international relations among States and other entities in respect of their activities both on and in relation to the oceans. In order to properly understand the systemic aspects of the international law of marine environmental protection, it is important to examine the cardinal principles of the international legal system in this field. In fact, the principles of the law have great potential value in three respects: (i) to integrate legal, economic and technological elements into a legal framework; (ii) to provide guidance in the interpretation and application of relevant rules; and (iii) to provide predictable parameters and the orientation for the development of law. While there is no generally agreed catalogue of principles governing marine environmental protection, this chapter will seek to examine in particular the five elements, i.e. the ‘no harm’ principle, the precautionary principle, the concept of sustainable development, the concept of common but differentiated responsibility and the principle of cooperation.