The concept of estoppel appears in Anglo-Saxon judicature, and also applies in international relations. It is current both in peace-time and in conditions of armed conflict. At its most general, one can define estoppel as a voluntary renunciation of procedures for establishing proofs. A physical or legal entity, especially however a state, cannot deny the existence of facts or of a law if previously they have drawn advantage from these facts. The author analyses different forms of the concept of estoppel: I. on the basis of concrete physical action; 2. on the basis of a treaty, compromise, exchange of notes, publication, or other written from; 3. on the basis of the principle of legitimacy.