The issue at the heart of Marbury v. Madison is the power of the judicial branch to interpret the U.S. Constitution. This case was decided a mere sixteen years after the adoption of the Constitution, and the idea that the letter of the law and the actual powers of government was still in question. The Articles of Confederation had failed as a government, and the ideas of separation of powers and checks and balances were just that- ideas.
Marbury v. Madison established the principle of judicial review; the idea that the job of interpreting the Constitution rested solely on the Supreme Court and the judicial branch. This idea was, of course, present in the Constitution, but Marbury v. Madision was the first time that power was executed. The power of judicial review is paramount in the balance of power in our government, and it manifested itself through this case.