This thesis attempts to study, in a comprehensive manner, the EEZ rule in both the LOS Convention and in state practice. Its central aim is to try to establish with exactitude the scope of the rule that has been taken into international custom. In this respect, after giving in chapter one a short exposé on the prevailing rules of the law of the sea that had governed all maritime spaces before UNCLOS III, serving as a background against which a better apprehension of the LOS Convention's EEZ provisions can be attained, an analysis of the rights of both coastal states and third states in the EEZ and their corresponding duties is provided in chapters two and three respectively. It has been asserted that, although a coastal state by claiming an EEZ would only enjoy specific functional rights, viz., the fields of activities they are connected with are explicitly defined, the vagueness often found in the wording of the Convention makes the situation not clear in all respects. While such a phenomenon may widen the functional limitations placed upon the general right of freedom of the high seas, it does not seem, however, to have any bearing on the high seas quality of the principal freedom of overflight, of laying cables and pipelines, and the freedom of navigation. Chapter four is a thorough examination and analysis of state practice as evidenced in EFZ and EEZ claims against the yardstick of LOS Convention. This is followed by a last chapter determining the scope of the rule that has been picked up in the new custom relating to the EEZ. In this connection, it is asserted that state practice gives strong evidence that a general right to claim a jurisdictional maritime zone as defined in Articles 55 and 57 of the LOS Convention, viz., extending seaward up to 200 miles from the baselines, is firmly established in international customary law.