(If you don't know what Marbury v. Madison was, just search for a brief somewhere)
In a nutshell, their reasoning was that the Judiciary Act of 1789 was unconstitutional.
William Marbury, a recently appointed judge by President John Adams before Thomas Jefferson became president, took Secretary of State James Madison to supreme court because Madison had been ordered by the new President Jefferson to not finalize paperwork for Marbury becoming judge.
Marbury asked the Supreme Court Justices to issue a writ of mandamus (basically an absolute order meant to remedy a situation) to make Madison finish the paperwork. This type of order was enabled from the Judiciary Act of 1789. It was up to the Supreme court now.
They were in a difficult situation of judicial restraint: on one hand they thought they should issue the writ of mandamus because they didn't want their decision to look like they were intimidated by the executive branch. But on the other hand, if they did, the executive branch would probably just ignore it.
So Chief Justice John Marshall did some thinking. The court came to the decision that the Judiciary Act of 1789 was unconstitutional because it gave more direct power to the Supreme Court than the constitution did. This decision was very clever because while it got the Judiciary branch out of that situation, it also gave them a different type of power; the ability to use the precedent from this case to rule something unconstitutional.
The Supreme court decision on Marbury version Madison by the federal judiciary. This is part of the court systems.
Marbury v. Madison
The effect of the landmark Supreme court decision in Marbury vs Madison helped in the separation of powers as far as the executive and legislature is concerned.
It gave the Supreme Court powers not given by the Constitution.
It gave the Supreme Court powers not granted by the Constitution
Marbury vs. Madison
It gave the Supreme Court powers not granted by the Constitution
Marbury vs Madison established the principle of "judicial review."Judicial review says the Supreme Court can decide on whether laws passed by Congress and signed by the President are constitutional.
marbury vs. Madison
Does the supreme court have the power to invalidate an act of congress because it violates the constitution.
It allowed the Supreme Court to overrule an unconstitutional law.
Marbury v. Madison, 5 US 137 (1803)Marbury v. Madison was an essentially meaningless case argued before the U.S. Supreme Court in 1803 in which the Court asserted the power of the Supreme Court to decide whether or not laws were constitutional. THAT is its lasting legacy, and every SCOTUS decision since cites Marbury v. Madison as their authority.For more information, see Related Questions, below.