 In order to correctly perform an international obligation, one must understand what it means and what it requires. The issue of interpretation is co-substantial with law and as a professional practice being a lawyer very often boils down to giving meaning to words and to sentences because law is only made of words. Much of what civil servants, attorneys and judges do in their daily life is ultimately about arguing and deciding on interpretations. There is no application of legal rules without interpretation and this is because very often interpretation precedes application or more fundamentally and even in the absence of any interpretative moment and endeavor identified as such. It is because applying a rule of law always conveys or if you prefer, betrays an understanding of that rule. Furthermore, interpreting law is never an abstract exercise even when it is done for the sake of the exercise. Interpretation is always about giving meaning to a rule, to an obligation or to a right in light of particular set of existing or potential facts. Interpretation is about giving flesh to a particular set of normative words in order to make them apply or not to certain facts. For the interpreter the question is whether the facts at hand come within the purview of the rule, such question leading to the need to read within the words used to phrase the rule and somehow see whether the facts are in it, the facts are covered by those words. Interpretation like law is not a natural science, very often various credible interpretations are possible and a choice between them will have to be made, or rather a choice which reflects preferences often lying outside the law will be reasoned and justified according to legal parameters, notably on the basis of the rules of interpretation. Moreover, interpretation does not take place in a vacuum, but interpretation is a social practice taking place between a certain breed of professionals which have been trained through law schools and practice to argue about the meaning of rules. What will be considered as a convincing interpretation will not be a matter of objective assessment but of collective judgment at a certain moment in time in a specific context. Interpretation will be admitted, a specific interpretation will be admitted as a good interpretation because it is shared and considered as making sense. The choice of a certain interpretation will be explained, notably by courts and tribunals, through a specific argumentation bridging the interpreted legal norm and the relevant rules on interpretation. In international law the issue of interpretation is no less present than in domestic law and in a way it is even more present because treaties are always the result of compromises. Treaties are, to use a famous sentence by Philippe Hallert, treaties are disagreement reduced to writing. And that writing is sometimes obscure or capable of various understandings because the drafters have resorted to so-called constructive ambiguities in order to reach an agreement. And the same questions about the purpose of what has to be achieved through interpretation will arise. What is interpretation about? Is it about faithfully finding the original intent of the author of the rule at the time the rule was designed? Or is it about discovering what should be achieved through the rule about what its purpose and raison d'être are? In other words, is interpretation backward-looking or forward-looking? Must it be historical or teleological? That tension is always present in the art of interpreting legal texts, and it is easy to explain that tension. On the one hand interpreting a legal rule is nothing more than saying what that rule is presumed to require from the very moment it came to exist. As a matter of principle, interpretation is said to be retroactive, and this conception is perfectly in line with the idea that judges do not make new laws, but only apply existing ones. But on the other hand, interpreting a legal rule is making sense of that rule today, in light of a particular set of facts and circumstances that could most probably not have been envisaged by the author of the rule at stake. The only rules on interpretation that exist in international law relate to treaty interpretation, and they are essentially provided for under Article 31 and 32 of the Vienna Convention. We shall look at those articles more closely in a moment, through various readings, but it is important at this stage to stress a few things about them. First, and as you will realize after having read those provisions, Article 31 and 32 strike a balance between the historical approach to interpretation and the theological approach. Second, Article 31 and 32 of the Vienna Convention are rules for interpreting treaties and treaties only, but because they are the only rules around on the interpretation of international law, those articles will also have some influence, sometimes even only as a matter of contrast, when it comes to the interpretation of other sources and instruments of international law. And we shall notably see that when discussing the interpretation of Security Council resolutions during the last week of the course. Third, it is now undisputed that Article 31 and also 32 of the Vienna Convention reflect customer international law, and can therefore be used to interpret any treaty. There are many cases in which courts and tribunals have affirmed that those provisions are declaratory of customer international law, but to take just one example, let me refer to the judgment of the International Court of Justice in a case between Botswana and Namibia about the boundary around an island on the Chobay River, which runs between the two African countries. The island is known as Casikili in Namibia and as Sedutu in Botswana, and the court in the judgment of 1999 finally ruled that the island is part of the territory of Botswana. The dispute between the two states was essentially about the meaning of a treaty concluded on 1st July 1890 between the United Kingdom and Germany, who were the colonial powers ruling over respectively Botswana and Namibia. Of course, when the treaty was concluded in 1890, the Vienna Convention of 1969 did not exist, and furthermore, neither Botswana nor Namibia are parties to the Vienna Convention when the dispute was decided by the court. But both disputing states referred to its provisions on treaty interpretation, and the court ruled as it did on many prior occasions. The court ruled that customer international law found expression in Article 31 of the Vienna Convention, and therefore, the court considered that Article 4 of the Vienna Convention, which says that the convention applies only to treaties concluded after its entry into force, the court said that this article did not prevent the court from interpreting the 1890 treaty in accordance with the rules reflected in Article 31 of the Convention. Let us turn now to the text of Article 31 and 32, and let us illustrate them through some other cases.