»International Commissions of Inquiry«: International commissions of inquiry are the subject of this treatise. Since the 1990s the United Nations have used such commissions increasingly to find facts and for legal assessment in situations in which violations of human rights and international humanitarian law are concerned. However, the concept of an international commission of inquiry is much older. Such commissions received an international legal framework for the first time at the 1899 peace conference at The Hague in the field of international dispute settlement. All types of international commissions of inquiry share a common feature: Their task is to find facts in an objective, independent and impartial manner. In the treatise, the applicable procedural law and procedural practice of such commissions are analyzed. The considerations are based on commissions which are formed by an agreement between states and commissions in the context of international organizations, namely the League of Nations and the United Nations.