Marbury vs. Madison was the first Supreme Court case to apply judicial review, which, under Article III of the Constitution, gave the Court power to void an act of Congress that was in violation of the Constitution. The decision helped define the Supreme Court as a separate branch of government
In 1803, Chief Justice John Marshall wrote the decision that played a key role in making the Supreme Court a branch with as much power as the legislative and executive branches. The case was complicated and it stemmed from the presidential election of 1800, which saw Thomas Jefferson defeat John Adams. In the last days of his presidency, Adams made numerous appointments for Justice of the Peace positions. The Senate approved the commissions and the President signed and sealed them but never delivered them.
When Jefferson took office he ordered his Secretary of State, James Madison, not to deliver them. One of those appointments was for William Marbury, who petitioned the Supreme Court to compel Madison to show cause why he had not delivered them. This direct petition was legal under the Judiciary Act of 1789.
Marshall ruled that the Court could not compel Madison do this because according to Article III of the Constitution, the only jurisdiction the Court had was in matters "affecting ambassadors, other public ministers and consuls" and "to cases in which the state shall be a party." The act of Congress was in conflict with the jurisdiction established in the Constitution and was, therefore, void.