Interstate Compacts are considered to be the "the most powerful, durable, and adaptive tools for ensuring cooperative action among the states."  The reasoning for this belief is that, unlike federally imposed mandates, interstate compacts are neither bogged down with stringent bureaucratic requirements nor are they backed by empty funds from a centralized source. Although there are a variety of reasons for an interstate compact to be enacted, the cause for their popularity is two-fold - they afford states the ability to act jointly outside the constraints of the federal government while at the same time respecting the concept of appropriate joint action imposed by Congress.
According to the Council of State Governments, from 1935-2010, about 150 interstate compacts have been created and the average state belongs to 25 of them. A spokesperson for the group says, "[I]nterstate compacts have typically been used...to stave off federal intervention into an area traditionally reserved for the states."
Though the precise motivations that spur the action for a joint contractual arrangement with two or more states differ depending on the state and vary from compact-to-compact. There are in general a set of uniform reasons that are all too frequent among a wide range of interstate compacts. First and foremost, interstate compacts offer a far more viable alternative to dealing with multi-state political matters than the stringent bureaucratic requirements of a federally imposed mandate. This precise motivation itself serves a dual-purpose - it provides " state-developed structure for collaborative and dynamic action" while at the same time builds consensus among the participating states to evolve in order to meet new and increased demands of the public in the long term.  Other rationale include the need of states to establish formal relationships with one another in order to address common problems or help promote similar agendas more effectively and efficiently, the desire among state agencies to codify their standard operating procedures with other states, the cost advantages of expansion to help reduce each state's administrative costs, and maintain the inherent right of states to sovereignty in matters traditionally reserved to them in the United States Constitution.
There is, however, an important caveat that downplays the desire for state to create a sense of uniformity among its neighbors. While compacts do require both houses of the state legislature and the governor to enact them, they are not uniform state law. This means to say that interstate compacts do not rely on contractual obligations. In other words, if a state has issues with any portion of the compact, it can alter it as it sees fit without the need for consent from other state members who had signed on with the agreement.
Unlike today, compacts were quite a rarity in the early years of the nation's history. Specifically, only 36 compacts were ever approved by the states between 1783 and 1920; those that were accepted dealt almost exclusively with boundary issues with neighboring states. Nowadays, however, there are over 200 such arrangements in existence and each state averages an involvement in 25 interstate compacts.
According to Article I, § 10, Clause 3 of the United States Constitution:
"No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay."
This became known as the Interstate Compact Clause. Some constitutional scholars, those who apply to the theory that the Constitution is a living document that must adapt itself to the current needs of the nation, contend that because it was adopted with no debate and has received merely cursory discussion among the Founding Fathers in the years after its passage, its full purpose and scope have never been fully developed. There are others, however, that cite the Tenth Amendment, which states that "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," as providing the states with the means in which to enter agreements with other states without needing the consent of Congress. An important caveat to this, however, is that such interstate compacts must not encroach on the powers of the federal government.
The Honorable Joseph Story, an Associate Justice on the United States Supreme Court from 1811 until 1845, was arguably one of the first legal authorities in the nation to theorize this principle. In his magisterial, Commentaries on the Constitution of the United States, he believed the clause was a means for the federal government to protect its supremacy over the states. 
Thus far, the Supreme Court of the United States itself has come to precisely the same conclusion. Two compact cases that were decided in the latter half of the twentieth century - New Hampshire v. Maine (1976) and Northeast Bancorp, Inc. v. Board of Governors of the Federal Reserve System (1985) - cited a late-eighteenth century Supreme Court decision, Virginia v. Tennessee (1893), in determining that the "application of the Compact Clause is limited to agreements that are ... 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."    A ruling from United States Court of Appeals for the Ninth Circuit in the Seattle Master Builders Association v. Pacific Northwest Electric Power and Conservation Planning Council (1986) stipulated further that congressional consent is not required for joint state activity not affecting federal authority.
The Political Consent Compact Clause Theory is the set standard by which the Supreme Court today bases its decisions regarding interstate compacts on. The crux of this argument is that determining "whether the compact contains a political subject affecting federal interests or the interests of non-compacting sister states, in which case congressional consent is required," is the sole basis on which the constitutionality of such agreements lies.  The 1978 Supreme Court case, United States Steel Corp. v. Multistate Tax Commission, deemed that the number of participating states into an agreement is entirely "irrelevant if it does not impermissibly enhance state power at the expense of federal supremacy, and the powers delegated to the administrative body must also be judged in terms of such enhancement." 
List of compacts
- ↑ 1.0 1.1 Council of State Governments - Understanding Interstate Compacts
- ↑ PoliticMo, "Missouri to consider joining interstate compact on health care", February 7, 2011
- ↑ The Founders' Constitution - Joseph Story, Commentaries on the Constitution 3:§§ 1396--99
- ↑ Justia - Virginia v. Tennessee, 148 U.S. 503 (1893)
- ↑ Justia - New Hampshire v. Maine, 426 U.S. 363 (1976)
- ↑ Justia - Northeast Bancorp, Inc. v. Governors, FRS, 472 U.S. 159 (1985)
- ↑ Connecticut General Assembly "Constitutionality of Interstate Compacts" 9 April, 2008
- ↑ Justia - United States Steel Corp. v. Multistate Tax Comm'n, 434 U.S. 452 (1978)