The theme of the paper is the United States Constitution, which is presented from a historical, institutional and functional point of view. First is described the historical development of constitutionalism in the United States, the creation of confederation, and then the federation. The adoption of the Constitution of 1787 created the first model of the federation in the world which was so effective and successfull that other countries tried to implement it. Then it is approached to the analysis of the provisions of the Constitution, its amendments, its principles and the founding institutions. The constitution establishes a divison of power between the legislative carried out by Congress, the executive led by the President and the judiciary performed by the Supreme Court together with the lower courts. The defined powers are assigned to each of them, each one is independent, but there is also a unique system of checks and balances that limit each other. A proportional system of federalism has also been established, in which powers between federations and member states are deployed. The last chapter refers to the role and importance of the Supreme Court which, in addition to its competences as a first instance and second instance court, plays the most important role in the control of the constitutionality and legality of all the regulations in the United States.