Universal jurisdiction is a fairly old concept in international law. However, during the 1990s some European states revived the principle in their legislations along with a judicial practice in a way that has, at times, been very controversial. Thus, universal jurisdiction became part of a heated debate between those who love it and those who hate it. Time had to pass to cool things off along with some setbacks in legislation and states' jurisdictional exercise of that principle. One reason of the controversy surrounding the principle of universality is, in the author's opinion, that the principle had a qualitative leap of rationale when applied in the late twentieth century. Thus, the first section of this chapter deals with the analysis of the concept of universal jurisdiction in criminal law; with particular emphasis on the evolution of its rationale. The second section deals with a modern conception of war crimes, regardless of the type of conflict in which they might occur.