What control does the Supreme Court have over Congress?
The US Supreme Court uses judicial review to declare laws or Acts of Congress to be invalid if they are contrary to the Constitution. The Constitution is the supreme law of the land and no presidential act or congressional laws may conflict with it. The Courts are the interpreters of the laws and as such they interpret the Constitution and laws to decide if they conflict with one another.
Further, it allows the Judicial Branch to "define" that law by answering questions about it that are not spoken to directly in the regulation itself.
Chief Justice John Marshall first used judicial review to declare an act of Congress unconstitutional in the case Marbury v. Madison, (1803), when the Court ruled Section 13 of the Judicial Act of 1789 assigned the Supreme Court powers not authorized by the constitution.
For more information, see Related Questions, below.