Under Article 38 of its Statute, the International Court of Justice can apply judicial decisions only as a ‘‘subsidiary means for the determination of rules of law’’. However, there are many reasons to believe that international courts and tribunals do play quite an important role in the progressive development of international law. There are a number of decisions which are inevitably recalled as the first step, or a decisive step, in the process of the formation of a new rule of customary international law. Can, in these cases, the judge be considered as a subsidiary of others? Are these cases compatible with the common belief that a judge cannot create law? Is this a peculiarity of international law, which is characterized by the existence of several courts but the lack of a legislator? Do decisions by different courts lead to the consequence of a fragmented international law? This volume aims to provide the reader with an elaboration of various questions linked to the legislative or, depending on the preferences, quasi-legislative role of courts. In their choices of subjects, the contributors have taken into account both the general aspects of the development of international rules through court decisions and the instances of specific sectors of international law, such as human rights, international crimes, international economic law, environmental law, and the law of the sea. Others have chosen the subject of rules on jurisdiction and the procedure of international courts. The question of the courts’ role, mutatis mutandis, in the development of areas of law different from public international law, namely private international law and European Union law, has also been considered.