The advent of the ICC as a permanent international criminal court to try and punish alleged perpetrators of the most serious crimes of concern to the international community as a whole is a crucial step forward in the architecture of international criminal justice. Nevertheless, as significant as the ICC is in the world today, we must remember that it is but one element in the broader global effort of eliminating impunity for international crimes. The role of the ICC in the framework of international criminal justice has accurately been referred to as that of a ‘court of last resort’. The permanence of the ICC ensures that there will always be a forum where perpetrators of international crimes may be held accountable; but this does not mean that all such crimes can or will be prosecuted before the ICC. The Rome Statute is built upon the premise that states have the primary obligation to take measures at the national level to ensure that the most serious crimes of concern to the international community as a whole do not go unpunished by investigating and prosecuting such crimes. Thus, national criminal jurisdictions are expected to take the lead in the fight against impunity.

The ‘principle of complementarity’ is one of the pillars of the system of international criminal justice created by the Rome Statute. While the principle of complementarity in its narrow sense refers to the admissibility of cases before the ICC, it has many other dimensions which are only beginning to be explored, such as ‘positive’ or ‘proactive’ complementarity. The present volume covers many of the different aspects of this fundamental principle. The authors offer insights into the theory and practice of complementarity, and the various contributions in this volume should lead to a better understanding of the principle of complementarity and its impact on the development of international justice. It is my hope that they will be a source of inspiration and provide invaluable insight for judges, practitioners, academics and researchers alike.