The United States Constitution provides the foundation for all of the laws in the United States. Therefore, all aspects of the criminal justice are governed and controlled by the guarantees in the U.S. Constitution. However, the early history of criminal justice system was relatively free from interference by the Supreme Court, even when it appeared that individual jurisdictions were engaging in behavior that may have violated the Constitutional rights of suspects or criminal defendants. During the past 50 years, the court has become more active and involved in the process of protecting Civil Rights and giving teeth to the rights guaranteed in the Constitution. This activism has led to three substantial changes in the criminal justice system. The first change is that criminal suspects must now be given verbal warnings, referred to as Miranda warnings, prior to custodial interrogation. The second change is that evidence obtained as a result of an illegal search, although not from the illegal search itself, is excluded under the fruit of the poisonous tree doctrine. The third major change is that the Constitutional prohibition against cruel and unusual punishment has been construed to prohibit the execution of juvenile defendants. .

             Prior to 1966, police departments were not required to give suspects verbal warnings prior to custodial interrogations. However, in Miranda v. Arizona the U.S. Supreme Court determined that the Fifth Amendment guarantee that U.S. citizens cannot be compelled to incriminate themselves lacked meaning if citizens were unaware of that right. As a result, the Court determined that a person in custody, prior to being interrogated, had to be informed of the right to remain silent. In addition, a person must be informed that if they waive their right to remain silent, the results of the interrogation can be used against them. Finally, a defendant must be made aware of his Sixth Amendment right to counsel.