The Constitution of the United States and the constitutions of the various states are the fundamental written law in those countries. The federal law must not violate the U.S. Constitution. All state laws must conform to or be in harmony with the federal Constitution as well as with the constitution of the state.
The U.S. Constitution is the basic document that gives authority to criminal justice agencies. It also sets the outer limits to their efforts, making sure that the system of crime control fits the form of government. All of the provisions of the Constitution apply to criminal control, though a number of amendments are system-specific. For instance, the Fourth Amendment prohibits illegal searches and seizures of evidence; the Fifth prevents self-incrimination during questioning; the Sixth guarantees the rights to jury trial; the Eighth provides for reasonable bail and prohibits cruel and unusual punishment; and the Fourteenth prevents from depriving anyone of liberty without due process of law.
Two very important principles of constitutional law are basic to the American judicial system. They are closely related to each other and known as the doctrine of separation of powers and the doctrine of judicial review.
The doctrine of separation of powers results from the fact that both state and federal constitutions provide for a scheme of government consisting of three branches — the legislative, the executive and the judicial. Separation of powers ascribes to each branch, a separate function and a check and balance of the functions of the other branches. The doctrine of separation of powers infers that each separate branch will not perform the function of the other and that each branch is summarized as follows: the Senate retains the power to approve key executive and judicial appointments. The legislative branch exercises control through its powers to appropriate funds.
In addition, Congress can limit or expand the authority of the executive branch or the jurisdiction of the judicial branch in most cases. The executive has the powers to appoint judges (in some states; the judiciary is elected). The judiciary has the powers to review actions of the executive and to review laws passed by the legislative branch to determine if such laws are constitutional.
The doctrine of judicial review is the heart of the concept of separation of powers. This doctrine and the doctrine of supremacy of the Constitution were established at an early date of the American history.