Article One of the United States Constitution
There have been a number of Supreme Court cases concerning what constitutes valid congressional consent to an interstate compact.  In Virginia v. Tennessee, , the Court found that some agreements among states stand even when lacking the explicit consent of Congress. (One example the court gave was a state moving some goods from a distant state to itself, it would not require Congressional approval to contract with another state to use its canals for transport.) According to the Court, the Compact Clause requires congressional consent only if the agreement among the states is "directed to the formation of any combination tending to the increase of political power in the States, which may encroach upon or interfere with the just supremacy of the United States".  The congressional consent issue is at the center of the current debate over the constitutionality of the not yet effective National Popular Vote Interstate Compact entered into by several states plus the District of Columbia.