The right of privacy is not spelled out in the Constitution. Some people say that one's right to privacy, "the right to be left alone," is so basic and so obvious that it did not need to be spelled out. They point to the Ninth Amendment, which says, "the enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people." In other words, just because the Constitution spells out some rights does not mean that citizens have given up all the other rights that are not spelled out and the Constitution does not look down on those unnamed rights. The Supreme Court does not base its notions of privacy on that amendment, however.

The best-known constitutional source for the right of privacy comes from the Fourth Amendment, which provides in part that, "the right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated." The Fourth Amendment is the source of the requirement that the police obtain a warrant before conducting a search of your possessions or your person (subject to several big exceptions), and it is usually thought of in that context. The First Amendment also may be viewed as source of a right to privacy in one's speech and association with others.

The Supreme Court uses the Due Process and Equal Protection Clauses in the Fourteenth Amendment to analyze questions involving a deprivation of fundamental rights, including the right to privacy. Those clauses guarantee that a state cannot "deprive any person of life, liberty, or property, without due process of law; nor deny to any person . . . the equal protection of the law."

The Supreme Court has used the fundamental rights analysis and the concept of privacy to invalidate state statutes that intrude too far into one's personal life. For example, the Court long ago invalidated a law that required students to attend public, rather than private, schools. The statute violated the Fourteenth Amendment because without a valid public interest, it restricted parents' freedom to make decisions about their children's education. In that case, which was decided early in the twentieth century, the law had the effect of precluding parents from sending their children to parochial schools. It followed an earlier decision that a state could not require that children be instructed only in English.

The Supreme Court has also protected privacy interests in matters of sexual reproduction. In a 1925 case, the court struck down a "three-strikes-and-you're-out" statute that authorized the sterilization of felons convicted of crimes of moral turpitude. The statute covered crimes like grand larceny, but not embezzlement, which is perhaps just a classier way of committing grand larceny. In the end, they are both theft. The Court concluded that marriage and procreation are fundamental to the existence and survival of the human race, and the state had no business interfering. This case does not prohibit all involuntary sterilization, however. Shortly before the court decided the three-strikes case, it decided that a young woman of very low intelligence could be involuntarily sterilized. One justice bluntly commented, "Three generations of idiots is enough."

In the 1960s, the Court decided Griswold v. Connecticut, 381 U.S. 479 (1965). At that time, Connecticut had statutes that prohibited the use of contraceptives and giving medical advice on how to use them. The Court invalidated the statute because it restricted the fundamental right of married people to make their own decisions about being parents, a state intrusion into the bonds of marriage. Justice Douglas, writing the majority opinion for the court, tried to find a specific basis in the language of the Constitution to support the result. He shied away from the Fourteenth Amendment. Instead, Justice Douglas wrote that the right to privacy came from "penumbras" and "emanations" of the Bill of Rights. Essentially, he saw a halo around the Bill of Rights and called it the right to privacy. As a result, the Court could enforce fundamental rights not listed in the Constitution against governmental intrusion. Justice Goldberg concluded that the right was based on the Ninth Amendment, and that to decide whether a right was fundamental, the Court should consider the "tradition and conscience" of the American people. Because marital privacy clearly was a historic value of Americans, the state of Connecticut had no business intruding. Other justices reached the same result using the Fourteenth Amendment.

The fundamental rights analysis also may be the basis for refusing to invalidate a law that could be seen as invading the right to privacy. One good example of this are two cases decided in 1997 that concluded that states could decide not to permit physician-assisted suicide for the terminally ill. People supporting the right to assisted suicide described it as allowing people to decide for themselves when they would to die. The Court, however, concluded that the right to die and assisted suicide simply were not fundamental rights. It noted that for centuries the law prohibited people from assisting another person's death by suicide. As a result, the "tradition and conscience" of the American people did not support a fundamental right to assisted suicide.