The Contracts Clause is found in Article I of the Constitution which states,
"No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility."
In the early history of our nation, this clause of the Constitution was respected and enforced. Unfortunately, as is the case with much of the Constitution, the trend of modern jurisprudence has been away from strict adherence and respect for the law toward a more subjective, personal, results-based approach. Observers of the judicial branch from all along the political spectrum, have accused judges and justices of ignoring the clear and plain language of the Constitution, or even intentionally misinterpreting that language, in order to reach their own desired outcome. That is, their own subjective idea of "justice". This is a movement away from justice under the law and the rule of law toward the capricious rule of men, but I am veering off topic.
You can read John O. McGinnis' article here.