The 10th Amendment states that "the powers not delegated to the United States by the Constitution, not prohibited by it to the states, are reserved...

“The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.”  --Tenth Amendment to the Constitution

When the Constitution was written, those against strong federal powers (called Anti-federalists), wanted to add amendments to guarantee individual liberties and the sovereignty of the states. The Tenth Amendment basically states that if a power was not mentioned in the Constitution for the federal government, it was reserved to the states or the people. In this way, the states would be guaranteed to keep many powers that they were accustomed to.

Because of this Amendment, most Americans believed that Congress had only the powers that were listed in Article 1, Section 8 of the Constitution. Most Americans failed to read the fine print. Clause 18 of that section states the following:

The Congress shall have Power ... To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.

John Marshall, as Chief Justice of the Supreme Court, reasoned that this stipulation in Article 1 enabled Congress more powers than maybe most citizens had previously thought.  The "necessary and proper" clause can be used to justify a wide range of actions by Congress.  This application of Section 8 has come to be known as the "implied powers" of Congress.  This does not stop citizens from challenging the actions of Congress or the president in court.  Ultimately, it is the responsibility of the courts to decide what an implied power is.