The US Supreme Court is the highest court in the land, meaning it has authority over all other courts within the United States. As there is no court in the United States with more authority than the US Supreme Court, a Supreme Court ruling cannot be overturned by any other court, though the Supreme Court can overturn its own rulings. This happens when a ruling is issued and then, years later, a similar case comes to the court and the court changes its opinion. One of the most famous examples of a Supreme Court case being overturned is Plessy v. Ferguson, which was overturned by the ruling in Brown v. Board of Education.
To understand how the US Supreme Court can overturn its own decisions, a person must first understand how the court works. Since 1869, the Supreme Court has consisted of nine judges, or justices, who serve for life. Unlike many other prominent government officials, Supreme Court justices are not elected. Instead, they are appointed by the sitting president. Presidents tend to select justices who share their political leanings, so the court can be more conservative or more liberal, depending on which presidents nominated the justices sitting on the court at any given time.
Most of the cases that come to the US Supreme Court are appellate cases. This means that the case has been heard in a lower court, but one of the parties in the case thinks the ruling made by the lower court is improper. That party appeals to the Supreme court to hear the case. The Supreme Court may rule on whether the lower court followed proper procedure or whether the laws in question are constitutional.
Plessy v. Ferguson is perhaps the most famous US Supreme Court case to be overturned. In 1890, a law in Louisiana required all railway companies to provide separate accommodations for black and white passengers. The parties in the case argued that requiring blacks and whites to ride separately violated the 14th Amendment to the Constitution, which states that no law can exist that violates the rights of a citizen of the United States. Following popular opinion at the time, the Supreme Court ruled that segregation was constitutional, as it did not prohibit black Americans from doing anything, but only asked that they do it in a dedicated location.
In the early 1950s, a similar case came to the Supreme Court. In Brown v. Board of Education, it was argued that segregating school children based on their race was unconstitutional. The social and political climate in the country had changed, and this time the court agreed and overturned its earlier ruling.