The guarantee of the Constitution lies definitively in the judiciary, because the written Constitution is law and the duty of the judge is to say what the law is. In addition, as the Constitution is the supreme law, judicial interpretation of the Constitution is supreme as well. Then, the very nature of the Constitution supreme, normative and written- involve the supremacy of the judiciary over other state and federal powers, because their interpretation of the Constitution prevail over any other-that it's judicial supremacy. In this paper we discuss this finding, in the context of American constitutional law. Analyzing the foundations of judicial supremacy is to demonstrate that the nature of the Constitution does not necessarily means that it is a court-finally, the federal Supreme Court-the ultimate interpreter of its meaning. The three characteristic features of the Constitution-written, normative and supreme, admit another reading, which leads not to the assertion of judicial supremacy as unavoidable.