International society has recently witnessed the emergence of two interrelated trends: on the one hand, the significant rise in transnational crimes, which constitute a serious challenge to States’ traditional security capabilities, and the noticeable increase in the number of international instruments put in place to bring that challenge to acceptable levels, on the other hand. The great emphasis the international community has put on the principle “aut dedere aut judicare” to strengthen the universal legal regime against serious crimes and to deny safe haven to their perpetrators is challenged not only by political, legislative and practical considerations pertaining to the lack of ratification and incorporation of international conventions and the wide disparity between States as to their implementation capacity, but also by the inherent differences between States’ differing legal systems. This is an area that, despite its relevance to international cooperation and human rights, has long been ignored by criminal justice comparative studies. Based on actual cases such as the Ramda and El Guerbouzi cases, this article examines and assesses the impact of the differences between civil law and common law systems on the effectiveness of international cooperation and human rights. It argues that, unless these differences are acknowledged and properly dealt with, perpetrators of serious crime will continue to constitute a serious threat to our peace and security.