Since the General Agreement on Tariffs and Trade (GATT) came into existence in 1948 (then later formed the World Trade Organisation (WTO) in 1994 at a conference of 123 countries), many economic blocs in the world have been established and it is the aspiration of many States to participate in a regional integration for a variety of different reasons such as language, religion, geography and increasing trade. One of the best examples of wide-ranging and advanced economic integration is the European Union (EU) which has established a Customs Union as well as an Internal Market. An advanced stage of Monetary Union (a Single Currency) has been adopted in the majority of its Member States. Of the many other examples of economic integration regulated by GATT, the Gulf Cooperation Council (GCC) may be considered as the best example of advanced regional integration in the Arab world as it has set up a Customs Union, a Common Market and there is a Monetary Union agreement among the majority of its Member States. On the basis of the foregoing, the thesis is a comparative analysis of the customs laws and practices of the GCC and of the EU. The comparison focuses on the nature and extent of the competences of the functionally equivalent institutions of these two organisations in relation to the implementation of the customs union and how and to what extent they are exercised – in order to evaluate the GCC system in particular and to suggest potential improvements. The thesis also explores the main obstacles that have faced the full implementation of the GCC customs union, including non-compliance by some of the GCC Member States with the agreed common commercial policy, with a view to providing suggestions and possible solutions for these obstacles. The thesis analyses whether the GCC has explicit or implicit international legal personality to act on behalf of its Member States at international level on issues related to the common commercial policy – as compared with the legal situation of the EU, pre- and post-Lisbon Treaty, by examining negotiation of the agreement between the GCC and the EU. The thesis also discusses the rules of international law related to conflict between treaties, in particular the Vienna Convention on the Law of Treaties 1969, in order to provide possible solutions for third party agreements which may conflict with GCC law.