This chapter conducts an examination of the formation, evolution, and contemporary challenges of international ocean rules. It centers on the codification process spanning from the Hague Conference to the Third UN Conference on the Law of the Sea and undertakes an analysis of the UNCLOS framework, encompassing its core characteristics, fundamental principles, and institutional mechanisms. Moreover, it also emphasizes the post-ratification developments and unresolved issues associated with the UNCLOS, including the CLCS, the ITLOS, the ISA, residual rights, unpredictably and implicitly regulated maritime activities maritime activities.