The court decisions that have the greatest impact—the ones that change American legal history—usually come from the United States Supreme Court. Since Supreme Court precedent binds lower courts (including state courts) on issues of federal and constitutional law, the decisions have a far-reaching impact.

Marbury v. Madison (1803)

One of the earliest cases decided by the Supreme Court, and arguably the most important, is Marbury v. Madison. The decision formally recognized the power of the judiciary to strike down laws that are contrary to the Constitution. The power of judicial review (that is, the power of judges to review and set aside the unconstitutional acts of the executive and legislative branches of government) gave life to the system of checks and balances that the Framers of the Constitution envisioned. Judicial review helps to ensure that laws passed in the heat of the moment, no matter how politically popular, do not undermine the basic framework of government or the individual rights that the Constitution protects.

McCulloch v. Maryland (1819)

Critics who believe that the federal government has too much power can trace their complaint to the Supreme Court’s resolution of McCulloch v. Maryland. By recognizing the federal government’s right to charter a national bank that the State of Maryland could not tax, the Supreme Court established a broad principle that makes it possible for the federal government to operate as we know it. The Court held that the federal government is not limited to the powers that are expressly stated in the Constitution, but may exercise implied powers that are necessary for the effective implementation of its enumerated powers. The Court also recognized that states, while retaining their own powers, are not authorized to interfere with the powers exercised by the federal government. McCulloch remains the key to the principle of federalism: the division of power between state and federal governments.

Santa Clara County v. Southern Pacific Railroad (1886)

The Supreme Court probably did not intend for this case to be pivotal. In fact, the tax issue that the case presents is forgettable. The case, nevertheless, illustrates the principle that remarks made by Supreme Court Justices tend to take on a life of their own. Before oral argument began, Justice Waite told the lawyers that the Court did not want to hear an argument about whether the Equal Protection Clause of the Fourteenth Amendment applied to corporations because all the justices agreed that it did. That observation gave birth to the legal doctrine that corporations have the same constitutional rights as individuals. The Court made that ruling explicit in a written opinion two years later.

The principle of "corporate personhood" has since been extended to the protection of corporate advertising as "commercial speech." More controversially, given the role of money in politics, the doctrine of corporate personhood underlies the ruling in Citizens United v. FEC (2010): that the First Amendment secures the right of corporations to influence elections by funding political broadcasts. One suspects that Justice Waite did not have that possibility in mind when he made his observation more than a century earlier.

West Coast Hotel Co. v. Parrish (1937)

The Constitution gives the federal government the power to regulate interstate commerce, but the extent of the government’s power to regulate business continues to generate debate among Supreme Court Justices and legal scholars. From the early years of the 20th century to the mid-1930s, the Supreme Court routinely struck down state and Congressional attempts to regulate business activities, to prohibit child labor, and to impose wage and hour requirements. The Court held that those laws impermissibly infringed upon freedom of contract. The Court’s decisions frustrated President Roosevelt’s New Deal legislation, as well as state reformers who wanted to regulate sweatshops.

After Roosevelt threatened to "pack" the Supreme Court by adding more justices, a key justice abandoned the Supreme Court’s "freedom of contract" reasoning and voted in West Coast Hotel Co. v. Parrish to accept the State of Washington’s power to enforce a minimum wage law. The decision ushered in a new era that recognized the power of government to engage in economic regulation. The case is also widely cited as a prominent example of the power of popular opinion to influence the Supreme Court’s decisions.

Brown v. Board of Education (1954)

The Civil War amendments to the Constitution ended slavery and guaranteed equal protection of the law to all races. Before the Brown decision, Plessy v. Ferguson (1899) interpreted the right to equal protection of the law to permit public facilities to be divided on the basis of race, provided that the facilities were comparable. The "separate but equal" doctrine was symbolized by separate drinking fountains for black and white people. Segregation was routinely practiced in schools, public bathrooms and railroad cars.

The Supreme Court finally rejected the notion that segregation is consistent with equality when it decided Brown v. Board of Education of Topeka, Kansas. Striking down a law that required white and black children to attend segregated schools, the Supreme Court recognized that racially divided facilities were rarely of equal quality. More importantly, segregation sent an unacceptable message that one racial group was inferior to another, a concept that is antithetical to the Fourteenth Amendment’s promise that laws must be applied equally, without regard to race.

Mapp v. Ohio (1961)

All people in the United States are protected by the Fourth Amendment’s prohibition of unreasonable searches and seizures. Evidence seized in violation of that constitutional right cannot be used in a criminal trial against the person whose rights were violated. That rule was not applied to state prosecutions before Mapp v. Ohio. The police broke into Dollree Mapp’s home without a warrant to look for gambling paraphernalia. They instead found "obscene literature." Mapp was convicted and sentenced for possessing that literature. The Supreme Court accepted Mapp’s argument that the "exclusionary rule" should extend to state as well as federal prosecutions. Mapp has since been relied upon as a precedent to extend other federal constitutional rights (such as the right to a jury and to confrontation of witnesses) to criminal prosecutions in state courts.

Engel v. Vitale (1962)

Some people mistakenly argue that Engel v. Vitale took away the right of students to pray in school. In fact, a student’s right to say a prayer that does not disrupt the classroom is protected by the Free Exercise Clause of the First Amendment. On the other hand, as Engel v. Vitale recognized, the Establishment Clause of the First Amendment prevents the government from leading students in an official prayer. The Supreme Court reminded Americans that the choice of whether to pray or not is an individual decision, not one that should be influenced by a school that chooses to endorse a particular religious belief by sponsoring a prayer. While the Engel decision was once controversial, the wisdom of separating church and state in a public school setting has come to be widely accepted as a core American value.

Gideon v. Wainwright (1963)

If you are charged with a crime, the Sixth Amendment provides that you have the right to be represented by a lawyer. However, until Gideon v. Wainwright, the right to counsel often meant that the accused had the right to hire a lawyer if he or she could afford one. Clarence Gideon was charged with burglarizing a pool hall and stealing money from a vending machine, a charge that he could not contest with the help of a lawyer, because Florida only provided appointed counsel in death penalty cases.

Recognizing that a defendant will rarely receive a fair trial if only the government is represented by a lawyer, the Supreme Court agreed with Gideon that the Sixth Amendment’s guarantee of the right to counsel includes appointed counsel for defendants who cannot afford to hire their own. By leveling the playing field for indigent defendants accused of crime, Gideon probably had a greater impact on criminal law than any other case in American legal history.

Griswold v. Connecticut (1965)

Although the Constitution does not use the words "right to privacy," nearly everyone agrees that certain fundamental decisions about life—whether to marry, whether to have children, the choice of an intimate partner, etc.—should usually be free from government intrusion. Griswold struck down a Connecticut law that prohibited the use of contraceptives. While Griswold has been criticized for inventing a right that does not expressly appear in the Constitution, most scholars agree that the values embodied in the Constitution encompass at least a limited right to privacy.

Few Americans would now argue that the government should have the power to interfere with such fundamentally private decisions as to whether to use birth control. Griswold became the foundation for one of the Supreme Court’s most controversial decisions, Roe v. Wade (1973), protecting a woman’s right to have an abortion; and for Lawrence v. Texas (2003), which recognized the right of gay couples to engage in "the most private human conduct, sexual behavior… in the most private of places, the home."

United States v. Nixon (1974)

In the nation’s early years, John Adams reminded Americans that they live within "a government of laws, and not of men." The notion that "no man is above the law," not even the president, was tested in United States v. Nixon. During the investigation of the Watergate scandal, a grand jury issued a subpoena to President Nixon, requiring him to produce tapes and documents concerning meetings with Watergate conspirators. President Nixon resisted the subpoena, arguing that "executive privilege" entitled him to defy the judiciary. In a unanimous decision, the Supreme Court rejected the notion that any government official, even one so powerful as the president, has the right to disregard laws that everyone else in the country must obey. The decision reaffirmed the supremacy of the rule of law in American government.