The Marbury vs. Madison case was so important because it was the first time the United States Supreme Court declared a law unconstitutional. The case established what is known as "judicial review."
The case took place in 1803, and the Supreme Court ruled that it could declare any act by Congress unconstitutional if the act violates or is inconsistent with the U.S. Constitution. The next case that would declare an act of Congress unconstitutional would not come about until 1857. The case, Dred Scott v. Sandford, was an attempt to establish the Missouri Compromise. In the 20th century, it became more common for the courts to strike down acts of Congress.