The Supreme Court is the most powerful court of law in the United States. It was authorized by Article III, Section 1 of the Constitution. It says, "the judicial Power of the United States, shall be vested in one supreme Court, and in such inferior Courts as the Congress may from time to time ordain and establish."

The court is made up of nine members, one chief justice and eight associate justices. Each is nominated by the president and must be confirmed by the Senate. Once a justice is appointed, he or she may serve for life. A justice can only be removed for serious wrongdoing. The current Supreme Court justices are John G. Roberts, Jr. (chief justice), Anthony M. Kennedy, Sonia Sotomayor, Antonin Scalia, Elena Kagan, Stephen G. Breyer, Clarence Thomas, Ruth Bader Ginsburg, and Samuel A. Alito, Jr. The death of Justice Scalia on February 13, 2016, left a vacant seat on the court.

The Supreme Court is the guardian of the Constitution. It has the power of judicial review. This means the court has the authority to overturn any act of government (local, state, or federal) that, in its opinion, violates the Constitution. Chief Justice John Marshall was the first to claim the power of judicial review. In Marbury v. Madison (1803), he made it clear that the Constitution, as the "superior, paramount law," is binding on all branches of government.

The Supreme Court heads the judicial branch of the federal government. The judicial branch shares power with the legislative branch (Congress) and the executive branch (headed by the president). By making the judicial branch independent, the Founders protected the courts from being dominated by either Congress or the president.

The court's rulings (decisions) are authoritative and final. They serve as guidelines for every other court in the nation. Supreme Court rulings can be changed only by constitutional amendment or by the Supreme Court itself.

According to Article III, Section 2, the Supreme Court is the first court to hear "all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party…." Such cases make up a category known as the court's original jurisdiction. In all other cases the Supreme Court exercises appellate jurisdiction to confirm or overrule a lower court's decision. The vast majority of cases heard by the Supreme Court come from U.S. appellate courts and the state supreme courts.

The first Supreme Court was established by the Judiciary Act of 1789. It consisted of a chief justice and five associate justices. For a period during the Civil War (1861–65), the court had ten members. In 1869 Congress fixed the size at nine, where it has since remained.

Every year the court hears fewer than 250 cases. This is less than 5 percent of the total number it is petitioned to review.

The annual sessions of the court begin on the first Monday of October. They run until late June. Oral arguments before the court are normally limited to an hour (a half hour for each side). During this time, the justices may ask questions. The parties may also submit briefs (summaries of the reasonings, legal and otherwise) that justify their position.

The justices hold conferences on Wednesdays and Fridays. They meet to discuss the cases they have just heard and indicate their initial decisions. At the end of the discussion, the justices vote. The chief justice, when voting with the majority, may write the majority opinion. (This task is often assigned to another justice in the majority.) If the chief justice holds the minority opinion, the senior justice voting with the majority will make the assignment.

Justices who agree with the conclusion of the majority opinion but not with its reasoning may write a concurring opinion. Those in the minority often write a dissenting opinion. Most decisions of the court are usually handed down on Mondays during the last few weeks of the term.

The Supreme Court has long upheld the authority of the federal government over the state governments. And since Gitlow v. New York (1925), the court has increasingly reviewed state legislation. Its mission is to prevent the states from violating the 14th Amendment by denying citizens equal protection or due process of law.  (For more information, see the article Bill of Rights.) 

Throughout its history, the court has had powerful critics. Presidents Thomas Jefferson, James Madison, and Andrew Jackson were extremely critical of the Marshall Court. They feared its decisions gave the federal government too much power. Abraham Lincoln attacked the decision rendered by Chief Justice Roger Taney in Dred Scott v. Sandford (1857). That law declared that African Americans could not become citizens of the United States. It also said that Congress did not have the constitutional authority to outlaw slavery in the federal territories. In Lincoln's view, the court's decision was not faithful to the Constitution. (For more information, see the article Dred Scott Decision.)

Perhaps the most famous confrontation between a president and the Supreme Court occurred in 1937, during the Great Depression. President Franklin D. Roosevelt was frustrated because the court had declared many of his key New Deal programs unconstitutional (often by a 5–4 vote). Roosevelt wanted to enlarge the court by appointing one additional justice for every sitting justice over the age of 70, up to a maximum of 15. This proposal was clearly designed to appoint six new justices favorable to the president's programs. However, this "court packing" legislation, as it came to be known, was decisively defeated in the Senate.

In recent decades, activists and traditionalists have disagreed as to how the court should interpret the Constitution. Activists argue that the court must look to the spirit of the Constitution when making its decisions. They believe that the Constitution is a living document. Therefore, its guidelines must be interpreted in light of the beliefs, needs, and circumstances of the day. Activists want interpretation to encompass ideas not specifically expressed in the Constitution.

Traditionalists maintain that the court should confine itself to the precise language of the Constitution, not its spirit. They argue that the court should try to determine what the Founders' intentions were and make its decisions accordingly. When this is not possible, traditionalists believe the court should defer to the judgment of the executive and legislative branches. They also think the court should overturn any state or federal law that is unmistakably contrary to the Constitution. Most of the justices now seated on the court are, to varying degrees, traditionalists.