Article III of the Constitution of the United States guarantees that every person accused of wrongdoing has the right to a fair trial before a competent judge and a jury of one's peers.

The Fourth, Fifth, and Sixth Amendments to the Constitution provide additional protections for those accused of a crime. These include:

• A guarantee that no person shall be deprived of life, liberty, or property without the due process of law
• Protection against being tried for the same crime twice ("double jeopardy")
• The right to a speedy trial by an impartial jury
• The right to cross-examine witnesses, and to call witnesses to support their case
• The right to legal representation
• The right to avoid self-incrimination
• Protection from excessive bail, excessive fines, and cruel and unusual punishments

Criminal proceedings can be conducted under either state or federal law, depending on the nature and extent of the crime. A criminal legal procedure typically begins with an arrest by a law enforcement officer. If a grand jury chooses to deliver an indictment, the accused will appear before a judge and be formally charged with a crime, at which time he or she may enter a plea.

The defendant is given time to review all the evidence in the case and to build a legal argument. Then, the case is brought to trial and decided by a jury. If the defendant is determined to be not guilty of the crime, the charges are dismissed. Otherwise, the judge determines the sentence, which can include prison time, a fine, or even execution.

Civil cases are similar to criminal ones, but instead of arbitrating between the state and a person or organization, they deal with disputes between individuals or organizations. If a party believes that it has been wronged, it can file suit in civil court to attempt to have that wrong remedied through an order to cease and desist, alter behavior, or award monetary damages. After the suit is filed and evidence is gathered and presented by both sides, a trial proceeds as in a criminal case. If the parties involved waive their right to a jury trial, the case can be decided by a judge; otherwise, the case is decided and damages awarded by a jury.

After a criminal or civil case is tried, it may be appealed to a higher court — a federal court of appeals or state appellate court. A litigant who files an appeal, known as an "appellant," must show that the trial court or administrative agency made a legal error that affected the outcome of the case. An appellate court makes its decision based on the record of the case established by the trial court or agency — it does not receive additional evidence or hear witnesses. It may also review the factual findings of the trial court or agency, but typically may only overturn a trial outcome on factual grounds if the findings were "clearly erroneous." If a defendant is found not guilty in a criminal proceeding, he or she cannot be retried on the same set of facts.

Federal appeals are decided by panels of three judges. The appellant presents legal arguments to the panel, in a written document called a "brief." In the brief, the appellant tries to persuade the judges that the trial court made an error, and that the lower decision should be reversed. On the other hand, the party defending against the appeal, known as the "appellee" or "respondent," tries in its brief to show why the trial court decision was correct, or why any errors made by the trial court are not significant enough to affect the outcome of the case.

The court of appeals usually has the final word in the case, unless it sends the case back to the trial court for additional proceedings. In some cases the decision may be reviewed en banc — that is, by a larger group of judges of the court of appeals for the circuit.

A litigant who loses in a federal court of appeals, or in the highest court of a state, may file a petition for a "writ of certiorari," which is a document asking the Supreme Court to review the case. The Supreme Court, however, is not obligated to grant review. The Court typically will agree to hear a case only when it involves a new and important legal principle, or when two or more federal appellate courts have interpreted a law differently. (There are also special circumstances in which the Supreme Court is required by law to hear an appeal.) When the Supreme Court hears a case, the parties are required to file written briefs and the Court may hear oral argument.