Who has the power to interpret laws?

All courts have the power to interpret laws relevant to cases before them. The US Supreme Court is the ultimate authority on federal and constitutional law.

Article III of the Constitution, which establishes the Judicial Branch, affords the Supreme Court the final authority to interpret laws, determine the constitutionality of laws, and to apply laws on the books to individual cases brought before it. The Supreme Court, in its authority to interpret laws, may make changes to laws brought before it if those laws are deemed unconstitutional or contrary to the legalities that were established by the laws when they were originally written.

Inferior courts may interpret and apply laws as they relate to everyday cases before them, but they cannot interpret any such laws in a manner that would defy rulings sent down by the Supreme Court.

For example, all courts must adhere to the legal interpretations of the Supreme Court and by the Constitution. The inferior courts are constrained by the decisions of the Supreme Court. Once the Supreme Court interprets a law, inferior courts must apply the Supreme Court's interpretation to the facts of a particular case. Ultimately the task of interpreting laws that are on the books falls on the United States Supreme Court, but most laws are written in such a way that they would only be challenged if there were an issue with a law that was deemed contradictory to the Constitution.

A good example of such a law that has come before the Supreme Court lately is the right for gay men and women to marry. Under the Fourteenth Amendment of the Constitution, people under US jurisdiction (not just American citizens) are guaranteed equal treatment/protection under the law. The case law of the Fourteenth Amendment is the case law that has been brought before the Supreme Court for interpretation.

Those parties who believe they have the right to marry whom they wish, man or woman, have argued to the Supreme Court that their Fourteenth Amendment rights have been violated as states outlaw gay marriage. Those parties against gay marriage have argued that marriage is a union between a man and a woman and always has been. There are problems for both sides because the Constitution makes no mention of homosexual marriage or its legality.

Another problem is the issue of state sovereignty, which allows a state to make its own laws without fear of reprisal from the Federal government. Since a concession cannot be reached, it is now the task of the Supreme Court to interpret the Fourteenth Amendment, weigh the arguments of both sides, and render a decision that falls in line with the Constitution, which ultimately has the final say, even though states do enjoy some freedom in making their own laws.