In this international Conference focused on the evolutions of legal concepts in the light of the evolution of international tribunals, the specific topic of Transitional Justice (TJ) and Impunity appears well integrated and deserves some remarks. The first remark is that during the last two decades the institutions of international criminal justice (ICTY, ICTR, ICC, and internationalized/hybrid criminal jurisdictions) have assumed a prominent role in the realm of international community and its law just in order to put an end to impunity for the perpetrators of the most serious crimes of concern of the international community as a whole (specifically war crimes, crimes against humanity, genocide, gross violations of human rights). Further remark is that the duty to prosecute such most serious crimes of international law is contained in some very important international conventions: the Genocide Convention (1948), the four Geneva Conventions (1949), the UN Torture Convention (1984). But we are able to affirm that nowadays this duty to prosecute most serious crimes has acquired a status of customary international law.