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In the United States, an interstate compact is a pact or agreement between two or more states, or between states and any foreign government. The Compact Clause (Article I, Section 10, Clause 3) of the United States Constitution provides that "No State shall, without the Consent of Congress,... enter into any Agreement or Compact with another State, or with a foreign Power,..."
The timing for Congressional consent is not specified by the Constitution, so consent may be given either before or after the states have agreed to a particular compact. The consent may be explicit, but it may also be inferred from circumstances. Congress may also impose conditions as part of its approval of a compact. Congress must explicitly approve any compact that would increase the states' political power in a manner that would encroach upon the federal government's power.
Most early interstate compacts resolved boundary disputes, but since the early 20th century, compacts have increasingly been used as a tool of state cooperation. In some cases, an agreement will create a new multi-state governmental agency which is responsible for administering or improving some shared resource such as a seaport or public transportation infrastructure.
Interstate compacts are distinct from Uniform Acts, which are model statutes produced by non-governmental bodies of legal experts to be passed by state legislatures independently, rather than constituting an agreement among multiple states.
Treaties between the states, ratified under the Articles of Confederation during the period after American independence in 1776 until the current U.S. Constitution was ratified in 1789, are grandfathered and treated as interstate compacts. This includes agreements like the Treaty of Beaufort, which set the boundary between Georgia and South Carolina in 1787, and is still in effect.
Operating agencies created by interstate compact
- Atlantic States Marine Fisheries Commission (Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, Virginia, North Carolina, South Carolina, Georgia, and Florida)
- Bear River Commission (Idaho, Utah and Wyoming)
- Bi-State Development Agency (Missouri and Illinois)
- Breaks Interstate Park Commission (Kentucky and Virginia)
- Colorado River Compact (Colorado, New Mexico, Utah, Wyoming, Nevada, Arizona, and California)
- Columbia River Gorge Commission (Oregon and Washington)
- Connecticut River Valley Flood Control Commission (Connecticut, Massachusetts, New Hampshire, and Vermont)
- Delaware River Basin Commission (Pennsylvania, Delaware, New Jersey, and New York)
- Delaware River Port Authority (Pennsylvania and New Jersey)
- Delaware River Joint Toll Bridge Commission (Pennsylvania and New Jersey)
- Delaware River and Bay Authority (Delaware and New Jersey)
- Dresden School District (New Hampshire, Vermont)
- Driver License Compact (all states except Georgia, Massachusetts, Tennessee, and Wisconsin)
- Eastern States Multi-state Council (New York, New Jersey, Pennsylvania, Massachusetts, Rhode Island, Connecticut, Delaware)
- Education Commission of the States (all states (except Washington), three territories, and Washington, D.C.)
- Emergency Management Assistance Compact (all states, plus Washington, D.C., Puerto Rico, and the U.S. Virgin Islands)
- Great Lakes Commission (Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania, and Wisconsin, plus Canadian provinces of Ontario and Quebec as associate members)
- Gulf States Marine Fisheries Commission (Alabama, Florida, Louisiana, Mississippi, and Texas)
- Interstate Commission for Adult Offender Supervision (all states, two territories, and Washington, D.C.)
- Interstate Commission on the Potomac River Basin (Maryland, West Virginia, Virginia, Pennsylvania, and District of Columbia)
- Interstate Compact on the Placement of Children (all 50 states)
- Interstate Environmental Commission (Connecticut, New Jersey and New York) 
- Interstate Oil and Gas Compact Commission
- Interstate Wildlife Violator Compact (all states except Hawaii and Massachusetts)
- Kansas City Area Transportation Authority (Kansas and Missouri)
- Midwest Interstate Passenger Rail Commission (Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Wisconsin)
- Multistate Tax Commission (all states except Delaware, Nevada, and Virginia)
- Northwest Power and Conservation Council (Oregon, Washington, Montana, and Idaho)
- Pacific States Marine Fisheries Commission (California, Oregon, Washington, Idaho, and Alaska)
- Palisades Interstate Park Commission (New York and New Jersey)
- Port Authority of New York and New Jersey (New Jersey and New York)
- Red River Compact Commission (Arkansas, Louisiana, Oklahoma and Texas)
- Regional Greenhouse Gas Initiative (Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Virginia)
- Rivendell Interstate School District (New Hampshire, Vermont)
- Susquehanna River Basin Commission (Pennsylvania, New York, and Maryland)
- Tahoe Regional Planning Agency (California and Nevada)
- Virginia-North Carolina High Speed Rail Compact (North Carolina and Virginia)
- Washington Metropolitan Area Transit Authority (Maryland, Virginia, and Washington, D.C.)
- Washington Metropolitan Area Transit Commission (Maryland, Virginia, and Washington, D.C.)
- Waterfront Commission of New York Harbor (New Jersey and New York)
- Western States Pact (California, Colorado, Nevada, Oregon, Washington) 
Non-operating interstate compacts
- National Popular Vote Interstate Compact, which will not take effect until additional states join the compact
- Northeast Interstate Dairy Compact, cancelled in 2001 by an act of Congress
- Transportation Climate Initiative, a proposed compact among the same states as the Regional Greenhouse Gas Initiative
|Wikimedia Commons has media related to Interstate compacts of the United States.|
- Enhanced cooperation, a similar system in the European Union
- "The Power of the States to Make Compacts". The Yale Law Journal. The Yale Law Journal Company. 31 (6): 635–639. April 1922. doi:10.2307/788529. JSTOR 788529.
- "Compacts Clause | Constitution Annotated | Congress.gov | Library of Congress". constitution.congress.gov. Retrieved 2020-04-14.
- "Interstate Compacts Fact Sheet" (PDF). csg.org. Lexington, Kentucky: National Center for Interstate Compacts. Retrieved April 19, 2020.
- "Atlantic States Marine Fisheries Commission: About Us". Archived from the original on 2004-04-27. Retrieved 2009-06-23.
- "Breaks Interstate Park Compact - Ballotpedia".
- Upper Colorado River Basin Compact of 1948, U.S. Bureau of Reclamation, 1948
- "Connecticut River Valley Flood Control Commission - Monitoring flood control dams in the CT River valley".
- csebestyen. "Delaware River Basin Commission-Milestones".
- "DRPA :: Delaware River Port Authority". Archived from the original on 2009-03-26. Retrieved 2013-08-11.
- Lahut, Jake (Apr 13, 2020). "New York Gov. Cuomo unveils multistate coalition to reopen economy after coronavirus". Business Insider. Retrieved July 26, 2020.
- "ECS Officers and President:ECS Officers for 2011-13". ECS Official Website. Education Commission of the States. c. 2011. Archived from the original on 2012-02-09. Retrieved 2012-02-19.
John Hickenlooper, Chair
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- "More Power for States: Good or Bad? | Bacon's Rebellion".
- "Tahoe Regional Planning Agency — TRPA". Tahoe Regional Planning Agency — TRPA.
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- "Home - WMATA".
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- "Waterfront Commission of New York Harbor (WCNYH)".
- Klar, Rebecca (April 27, 2020). "Colorado and Nevada join western states cooperating on reopening". The Hill. Retrieved July 26, 2020.