The most important fundamental principle in international law is that of good faith. This is upheld by the Charter of the United Nations (UN) in the following terms : “all Members, in order to ensure to all of them the rights and benefits resulting from membership, shall fulfil in good faith the obligations assumed by them in accordance with the present Charter”.
This was subsequently reinforced by the UN General Assembly  which declared that States are obliged to fulfil in good faith their obligations deriving from international law generally, including treaties. Accordingly, the Vienna Convention on the Law of Treaties codified international customary law when established that every treaty in force is binding upon the parties to it and must be performed by them in good faith .
The same Convention goes on to apply the same principle to the general rules of interpretation. This is done in the following words: “a treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose “. The Convention advances further on the principle of good faith when dealing with the matter of overriding provisions of internal law, deemed to be ilegal, unless under special circumstances .
Contrariu sensu, the Convention also upholds the requirement of good faith in negotiations leading to a treaty when it admits the invalidation of its contents in the cases of error , fraud , corruption , coercion  and conflict of treaties . This is only natural as international law must discourage the use of force in international relations, by means of upholding and enshrining principles such as consent and good faith. Thus, the principle of good faith has become a peremptory norm of general international law.
Accordingly, the International Court of Justice (ICJ) decided that: “one of the basic principles governing the creation and performance of legal obligations, whatever their source, is the principle of good faith. Trust and confidence are inherent in international co-operation, in particular in an age when this co-operation in many fields is becoming increasingly essential “.
Of course, the principle is also applicable to multilateral trade law, which has a lower hierarchical standing  in comparison with the UN Charter and the Convention. In this area, the duty of care, often defined as special and differentiated treatment, is added when the legal trade order recognises the position of vulnerability of developing countries.
Thus, the General Agreement on Tariffs and Trade of 1947 (GATT 47), which is still in force, recalls that the basic objectives of the agreement include the raising standards of living and the progressive development of the economies of all contracting parties. It also notes the character of urgency in the attainment of these objectives by developing countries . The agreement goes on to establish that there is a need for a rapid and sustained expansion of the export earnings of the developing countries .
In the matter of the General Agreement on Trade in Services (GATS), one of the treaties of the Uruguay Round concluded in 1994, the principle of good faith becomes even more important. This is so because the GATS does not contemplate safeguards , a trade remedy used for the protection of the domestic sector in case of damage, injury in the trade jargon , or threatened damage to the producers of like or directly competitive products.
Therefore, the GATS contemplates the increased participation of developing countries by means of concessions from developed countries with a view to:
a) strengthening of their domestic services capacity and its efficiency and competitiveness;
b) the improvement of their access to distribution channels and information networks; and
c) the liberalisation of market access in sectors and modes of supply of export interest to them .
In addition, developed country members are to provide developing country Members with information related to their respective markets concerning:
a) commercial and technical aspects of the supply of services;
b) registration, recognition and obtaining of professional qualifications; and
c) the availability of services technology .
On the other hand, the Understanding on Rules and Procedures Governing the Settlement of Disputes (DSU), another of the treaties of the Uruguay Round, when dealing with the matter of consultations, determines that members should give special attention to the problems and interests of developing countries .
Based on the foregoing, it results eminently clear that when a developed country member state of the World Trade Organization (WTO) demands concessions from a developing country member state in connection with the GATS, the test of good faith would require:
a) that the developed country provides the developing country (ies) with full disclosure of information pertaining to their own markets;
b) that the developed country provides the developing country(ies) with a bona fidae analysis of how such a concession would strengthen its domestic industry and how the international access of such industry would be ameliorated; and
c) that the developed country provides the developing country(ies) with a statement on how it took into consideration the problems and interests of the developing country(ies) when making the demand in reference.
Lawyer admitted in Brazil, England and Wales and Portugal. GATT and WTO panelist. Brazilian government ad-hoc representative for the Uruguay Round of the GATT. Post-graduation professor of the law of international trade.