How can deontologists reconcile the stringency of the moral prohibition on torture with the recognition that it may sometimes be the only means to prevent catastrophe? This article proposes a conception of deontology that allows for the resolution of this dilemma. Casting deontology in terms of the distinction between categorical and conditional obligations, we articulate a theoretical distinction between principles and exceptions. Torture can never be permitted or authorized by a rule or principle. Whenever it is justifiably performed it is performed under urgent and exceptional circumstances. On this basis the practical necessity of torture may be acknowledged, while the categorical nature of the prohibition on torture is maintained. This explains how torture might be categorically prohibited by international law, yet at the same time be subject to a defence of necessity or duress under international criminal law.