Universal jurisdiction is a type of extraterritorial jurisdiction in international law. It permits any State to prosecute persons accused of committing certain abuses regardless of where the offence occurred and irrespective of the nationality of the accused person(s) or victim(s). The inhumanity of the act demands that the perpetrator be prosecuted, because he/she violates the common interests of the world community. As such, it is a rationale-based jurisdiction. In recent years, States that exercise this jurisdiction have narrowed the scope of its application in their legal systems, by necessitating a link between the crime abroad and the State prosecuting the offence. This nexus is required through the nationality or residency of the alleged perpetrator(s) and / or victim(s). Moreover, in some States the powers of the public prosecutor in deciding whether or not a criminal investigation proceeds makes it impossible for victims of international crimes to seek justice. The result is a significant change in the exercise of universal jurisdiction that could potentially blur the distinction between it and other forms of extraterritorial jurisdiction. This situation raises an important question as to whether the reforms are in conformity with the underlying rationale for universal jurisdiction. What is more, it must be recalled that the primary responsibility for the prosecution of serious crimes under international law rests with States. From the point of view of preventing impunity, it is important that the modern rationale of universal jurisdiction is identified, given that the other principles of jurisdiction in international law, namely the territorial, active and passive principles are not as far reaching as the universality principle. This doctoral research does three things. First, it provides a comprehensive analysis of the rationale for the development of universal jurisdiction over the crimes to which it applies. It should be noted that the crimes within the scope of the thesis are those that are deemed ...