The fifty American states are separate sovereigns, with their own state constitutions, state governments, and state courts. All states have a legislative branch which enacts state statutes, an executive branch that promulgates state regulations pursuant to statutory authorization, and a judicial branch that applies, interprets, and occasionally overturns both state statutes and regulations, as well as local ordinances. They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the federal Senate. Normally, state supreme courts are the final interpreters of state constitutions and state law, unless their interpretation itself presents a federal issue, in which case a decision may be appealed to the U.S. Supreme Court by way of a petition for writ of certiorari.State laws have dramatically diverged in the centuries since independence, to the extent that the United States cannot be regarded as one legal system as to the majority of types of law traditionally under state control, but must be regarded as 50 separate systems of tort law, family law, property law, contract law, criminal law, and so on.

Most cases are litigated in state courts and involve claims and defenses under state laws.In a 2012 report, the National Center for State Courts' Court Statistics Project found that state trial courts received 103.5 million newly filed cases in 2010, which consisted of 56.3 million traffic cases, 20.4 million criminal cases, 19.0 million civil cases, 5.9 million domestic relations cases, and 1.9 million juvenile cases. In 2010, state appellate courts received 272,795 new cases. By way of comparison, all federal district courts in 2010 together received only about 282,000 new civil cases, 77,000 new criminal cases, and 1.5 million bankruptcy cases, while federal appellate courts received 56,000 new cases.