The criminal justice process is the process by which crimes are investigated, charged, tried, and punished. Because the criminal justice system involves actions by the government against its citizens, the criminal justice process has been the subject of constitutional protections dating back, in the English common law system, to the Magna Carta.

The constitutional protections afforded to citizens under modern law derive from both federal and state constitutions. The U.S. Constitution sets forth numerous principles governing both federal and state criminal proceedings.

• In state proceedings, federal constitutional principles set a baseline standard of protection of the rights of persons accused of crime.
• Additionally, many state constitutions give accused persons additional or more extensive rights than provided by the federal constitution. In those states, these higher standards must also be met in criminal prosecutions.

In addition to adhering to constitutional standards, the criminal justice systems in the states and the federal government are defined by statutes and court rules, which set forth the manner in which crimes are investigated and prosecuted. These statutes and court rules typically vary greatly from state to state.

The process of investigating a criminal case often involves a search for evidence of the crime. The Fourth Amendment to the U.S. Constitution places significant limits on the government’s ability to search the homes and property of citizens. Under the Fourth Amendment, law enforcement officers must have a search warrant, issued on a showing of probable cause, in order to search a private home or premises without the consent of the occupants.

However, there are numerous limitations on the requirement that officers obtain warrants prior to search. In certain limited, "exigent" circumstances, police may search a home or other premises without first obtaining a warrant. These circumstances include a good faith belief on the part of the police that:

• a person’s life is in danger
• evidence might be destroyed
  or
• a suspect may be about to escape

If a person is arrested, officers may search the individual and the immediate area around the person, in a search "incident to an arrest."

Electronic eavesdropping is regarded as a form of search under the Fourth Amendment. As a result, law enforcement officers must obtain warrants before tapping phones. These warrants are typically issued with very tight limitations on:

• the time period in which they can be conducted
• the manner in which law enforcement officers must limit eavesdropping on conversations not related to criminal activity

Under the judge-made "exclusionary rule," evidence that is seized in violation of the Fourth Amendment cannot be introduced as evidence in the trial of a criminal case. The purpose of this rule is to dissuade law enforcement officers from ignoring Fourth Amendment limitations on their right to search in order to obtain evidence of a crime.

The Sixth Amendment to the U.S. Constitution guarantees the right to legal counsel at all significant stages of a criminal proceeding. The right to legal counsel is considered so important that there is an associated right given to persons, who are indigent and unable to pay for legal counsel, to have counsel appointed and paid for by the government. Both the federal criminal justice system and the individual state systems have procedures for the appointment of counsel for indigent defendants.

The Sixth Amendment right to counsel has been extended to:

• the interrogation phases of an investigation
• the trial
• sentencing proceedings
• at least an initial appeal of any conviction

The Fifth Amendment to the U.S. Constitution provides that no person may be compelled to be a witness against himself or herself. This relatively simple principle has numerous applications in the law of criminal procedure.

• Under the Fifth Amendment and under most circumstances, a person may refuse to answer the questions of law enforcement officers. If a person is called before a grand jury to testify, the person may refuse to answer any questions posing the possibility that they will incriminate themselves. This means that a person can refuse to answer a question requiring them to admit some action constituting a crime or subjecting them to criminal prosecution. Prosecutors may give such individuals immunity from prosecution in order to compel them to testify despite their Fifth Amendment right.
• The Fifth Amendment also means that criminal defendants cannot be called as witnesses at their own trials without their consent.

The term "Miranda rights" comes from a U.S. Supreme Court case, Miranda v. Arizona. In that case, the Supreme Court held that suspects in the custody of law enforcement officers must be specifically informed that they have certain constitutional rights before they are questioned. The purpose of this ruling is to make sure that if an accused person answers questions in an interrogation, that this "waiver" of their constitutional right to remain silent or to have an attorney present is a "knowing waiver." A "knowing waiver" of a right is one that is made freely, with knowledge of the existence of the right.

The Miranda rights include:

• the Fifth Amendment right to remain silent
• the Sixth Amendment right to an attorney

These rights, familiar from endless television police dramas, are typically given as follows:

"You have the right to remain silent. If you give up the right to remain silent, anything you say can and will be used against you in a court of law. You have the right to an attorney. If you cannot afford an attorney, an attorney will be appointed for you."

The exclusionary rule, a judge-made rule limiting the actions of law enforcement officers, prohibits the use of evidence obtained in violation of a person’s Miranda rights in a criminal prosecution. This means that if a suspect is not given Miranda rights, or if those rights are otherwise violated, any confession given by the suspect may not be used at a trial on the charges.

The right to a public trial by jury is one of the oldest protections given to citizens against government law enforcement efforts.

• At common law, a criminal defendant is entitled to a public trial by jury in cases of felonies and in cases of misdemeanors where the term of imprisonment may exceed one year.
• Under current constitutional law principles, a jury trial is required where the term of imprisonment may exceed six months.
• Individual states may provide for a jury trial in a broader class of criminal cases.

In the criminal law context, to be put "in jeopardy" means to be brought to trial for an offense. Under the Fifth Amendment, a person may not be "twice put in jeopardy" for the same crime. This principle means that law enforcement cannot have two bites at the apple, and if a person is found not guilty of a crime, they cannot be tried again for the same crime.

There are some limitations to this principle.

• A person who is found not guilty of a crime under state law may, under certain circumstances, be tried for the same conduct under federal law.
• Conduct constituting a crime may amount to a negligent or intentional injury of another person. It is not considered being put "in jeopardy" for a person to be sued under the civil law. Because the proof requirement in a criminal case is higher (proof beyond a reasonable doubt) than in a civil case ("preponderance of the evidence"), an injured person can seek damages for that injury under the civil law of torts. The suit can be made, regardless of whether the defendant has been found not guilty of a crime based on the same set of facts. .