What does the supreme court do when they exercise judicial review?

The term "judicial review" refers to the practice of evaluating laws and policies relevant to a case before the court to determine whether they're in compliance with the US Constitution.

Historically, the Supreme Court's role rested in formally securing the right of judicial review for the Judicial Branch, which was accomplished in Chief Justice John Marshall's opinion for Marbury v. Madison, (1803).

Marshall's idea wasn't new; judicial review is a longstanding practice arising from English common law and had already been adopted by the US courts. In his opinion for Marbury, Marshall described a fairly recent incident involving a 1796 Act of Congress concerning military pensions that a federal Circuit Court had declared unconstitutional. Congress accepted the decision without argument, and made the required changes to the legislation.

The Supreme Court had also used judicial review in earlier cases, such as Hylton v. Ware, (1796), when the Ellsworth Court evaluated the constitutionality of a federal treaty and upheld its provisions against the state. Most people overlook this case because the federal law was deemed constitutional and didn't result in a conflict between different branches of government.

Today, the Supreme Court is established as the final authority on interpreting laws and assessing their constitutionality.