The prevalence of rape and its widespread impunity, whether committed during armed conflict or peacetime, has been firmly condemned by the UN and its prohibition has been consistently recognised in international law. This development, however, is a rather novel endeavour. The belated response is in part a consequence of rape being characterised by such myths as sexual violence representing an inevitable by-product of war or as being committed by sexual deviants. Its systematic nature has thus been ignored as has the gravity of the offence, often leading to a culture of impunity. This was evident, for example, through the failure to prosecute crimes of rape during the Nuremberg trials, in qualifying it as a harm against a woman’s honour in the 1949 Geneva Convention (IV), or in considering it a violation located in the “private sphere”, thereby beyond regulation by international law. However, substantial efforts have been made in international law to recognise obligations for states to prevent rape. A prohibition of the offence has developed both through treaty law and customary international law, requiring the prevention of rape whether committed by state agents or by a private actor. One measure to prevent such violence has been identified as the duty to enact domestic criminal laws on the matter. The flexibility for states in determining the substance of such criminal laws is increasingly circumscribed, leading to the question of whether a particular definition of rape or certain elements of the crime must be adopted in this process. Elaborations on the elements of the crime of rape have been a late concern of international law, the first efforts made by the ad hoc tribunals (the International Criminal Tribunal for Rwanda and the International Criminal Tribunal for the Former Yugoslavia), followed by the regional human rights systems as well as the International Criminal Court. The principal purpose of the thesis is consequently the systematisation and analysis of provisions and emerging norms obliging states to adopt a particular definition of rape in domestic penal codes. The prohibition of rape and, subsequently, the process of defining the crime has been made in three areas of international law – international human rights law, international humanitarian law and international criminal law. Emerging norms in all three regimes are consequently examined in this thesis, bringing to the fore overarching questions on the possible harmonisation of defining rape in these distinct branches of international law. The study will thus provide a contextual approach, aiming to evince whether the definition can be harmonised or if prevailing circumstances, such as armed conflict or peace, should necessarily inform its definition. Ultimately, the advances in international law are evaluated in order to identify possible areas for further development.