García Amador, the first Special Rapporteur on the topic of state responsibility, was of the view that in international law, special circumstances denote situations of exemption from the secondary rule which attaches disadvantageous consequences to breach of obligation under any primary rule. By contrast, his successor, Roberto Ago, advanced the notion that the exceptional circumstances characterise situations of exemption from the obligations arising out of primary rules. In departure from both conceptions, James Crawford, the last Special Rapporteur on the law of state responsibility, submitted that it is only after having established that a certain course of behaviour is contrary to the terms of a prohibition that it becomes possible to enquire whether the actor had a justification for acting as he did. If affirmative, the conduct in question is to be regarded as conduct which neither conforms with the obligation nor constitutes a violation thereof. If negative, then the act amounts to an unjustified wrongdoing and accordingly forms the basis for the operation of the various disadvantageous consequences envisaged by the law of international responsibility. This contribution first examines the process of the development of the notion of exceptional circumstances in the edifice of the law of international responsibility ex delicto. It then shows that from the three competing conceptions, only that advanced by Ago proves capable of accurately accounting for the circumstances of consent, self-defence, and the legitimate application of a countermeasure.