The purpose of this paper is to study the judicial activism of the courts in the following systems: France (III), the United States (IV) and the European Union (V). As we will see later, the courts used judicial review as a powerful tool to interpret (II) the constitutions or treaties. It helps them in the tasks that were assigned to them by the different Constitutions, or treaties as far as the European Union is concerned. I will present how the courts enhanced their own power of judicial review. For the United States is was more a matter of creation, where in France and in the European Union, the courts merely improved the powers already delegated to them by the texts. However in those three systems the courts showed originality and determination in their search for tools. The courts did not just try to gain more power, they tried to find in the texts and in their spirit the justifications for the judicial review power they developed. The goal of the courts was to possess an efficient tool that would allow them to interact with the other institutions and follow the evolution of the structure they were in.