The U.S. Constitution separates the U.S. government into legislative, executive and judicial branches. Each branch has its own set of powers and responsibilities. The legislative branch passes laws; the executive branch -- headed by the President of the United States -- can either sign or veto laws passed by Congress. However, even when a law is passed and signed, the judicial branch can nullify it by declaring it unconstitutional.

Article III

The judicial branch of the U.S. government is established by Article III of the U.S. Constitution. Article III established the Supreme Court and gives Congress the authority to establish lower federal courts as necessary. Supreme Court justices and other federal judges are appointed for life and can only be removed from office by resignation, impeachment or death. The federal court system established by the Constitution has jurisdiction over all cases between foreign interests and U.S. citizens, between U.S. states and citizens and between different states. The federal courts also hold appellate authority in all legal cases within the United States.

Checks and Balances

The U.S. government is designed such that the three branches of the federal government have a number of built-in checks and balances. The judicial branch can declare laws passed by Congress or executive acts by the President of the United States unconstitutional, thus legally voiding them. However, all federal judges are nominated by the president and approved by the Senate. Additionally, Congress can remove a federal judge from office by impeachment.

Judicial Review

The process by which the U.S. judicial branch declares a law unconstitutional is called judicial review. The Constitution does not actually spell out the power of judicial review. Rather, it is known as an "implied power." The U.S. Supreme Court firmly established its use of judicial review in the 1803 case of Marbury vs. Madison, in which the Supreme Court overturned part of the Judiciary Act of 1789.

Federal Court System

The judicial branch of government is further broken down into federal district courts, appeals courts and the Supreme Court. There are a total of 94 judicial districts, including at least one in each U.S. state. These courts try both civil and criminal cases. These judicial districts are grouped into 12 regional circuits, each of which is represented by a Court of Appeals. Cases involving the constitutionality of laws generally start in state or federal courts. They may then advance to the Courts of Appeals or the U.S. Supreme Court through the appellate process. Decisions made by the Supreme Court are the ultimate interpretation of the laws of the United States.