Abstract Induced by the work of the Institute of International Law on the limits to evolutive (dynamic) interpretation of constituent instruments of international organizations, the author discusses the issues of evolutive and dynamic interpretation, limits to the interpretation, legitimacy of informal modification of constituent instruments and borderline between interpretation and informal modification. He will propose conditions under which an informal modification may be legitimate. In spite of the fact that the borderline between interpretation and informal modification cannot be precisely located, the author will refer to certain judicial indications that its location may depend on the way by which a constituent instrument responds to the disputed issue. The author will turn attention, also, to the meeting points of judicial interpretation and informal modification.