Implied powers, in the United States, are powers authorized by the Constitution that, while not stated, seem implied by powers that are expressly stated.
When George Washington asked Alexander Hamilton to defend the constitutionality of the First Bank of the United States against the protests of Thomas Jefferson, James Madison, and Attorney General Edmund Randolph, Hamilton produced what has now become the classic statement for implied powers. Hamilton argued that the sovereign duties of a government implied the right to use means adequate to its ends. Although the United States government was sovereign only as to certain objects, it was impossible to define all the means it should use, because it was impossible for the founders to anticipate all future exigencies. Hamilton noted that the "general welfare clause" and the "necessary and proper clause" gave elasticity to the Constitution. Hamilton won the argument with Washington, who signed the bank bill into law.
Later, directly borrowing from Hamilton, Chief Justice John Marshall invoked the implied powers of government in the United States Supreme Court case, McCulloch v. Maryland. In 1816, the United States Congress passed legislation creating the Second Bank of the United States. The state of Maryland attempted to tax the bank. The state argued the United States Constitution did not explicitly grant Congress the power to establish banks. In 1819, the Court decided against the state of Maryland. Chief Justice Marshall argued that Congress had the right to establish the bank, as the Constitution grants to Congress certain implied powers beyond those explicitly stated.
In the case of the United States Government, implied powers are powers Congress exercises that the Constitution does not explicitly define, but are necessary and proper to execute the powers. The legitimacy of these Congressual powers is derived from the Taxing and Spending Clause, the Necessary and Proper Clause, and the Commerce Clause.
Implied powers are those that can reasonably be assumed to flow from express powers, though not explicitly mentioned.