In the United States, the power to define crimes and set punishment for them rests with the legislatures of the United States, the several states, and the territories, the principal authority being that of the individual states. This power in the states is restricted by the federal Constitution, e.g., in the Fourteenth Amendment and in prohibitions against acts of attainder (an act of attainder is a legislative declaration that a particular individual is guilty of a crime) and against ex post facto laws (laws that retroactively declare certain actions to be criminal). State constitutions may also limit state legislative action. The courts cannot look further into the propriety of a penal statute than to ascertain whether the legislature has the power to enact it. Administrative rules may have the force of law, and violations of such rules are punishable as public offenses, provided that the legislature has made such violations misdemeanors.
A common-law crime is one punishable under common law, as distinguished from crimes specified by statute. In many U.S. jurisdictions, including some in which comprehensive criminal statutes have been enacted, the common law in relation to crimes and criminal procedure has been recognized by the courts as in force, except insofar as it has been abrogated or repealed, expressly or impliedly, by statute. Thus the state may prosecute crimes that were indictable at common law even though they may not be denominated as such or be provided for by statute. In many other jurisdictions the courts have held the common law as to crimes as being abolished, and no act is punishable as a crime unless it is made so by statute, or unless the act is made punishable as a crime by the constitution. Criminal procedure is entirely regulated by statute. There are no common-law offenses against the United States, and one may be subject to punishment for crime in a federal court only for the commission or omission of an act defined by statute or regulation having legislative authority, and then only if punishment is authorized by Congress. In general, crimes must be defined in a penal statute with appropriate certainty and definiteness; the constitutional requirement of due process of law is violated by a criminal statute that fails to give a person of ordinary intelligence fair notice that the contemplated conduct is forbidden by the statute.
Except as otherwise provided by statute, to constitute a crime an overt act (actus reus) must be accompanied by a criminal intent (mens rea) or by such negligence as is regarded by law as equivalent to a criminal intent. Motive, or that which leads or tempts the mind to indulge in a criminal act, as distinguished from intent, is neither a crime nor an essential element of a crime. The motive with which an offense was committed is immaterial. Proof of motive may be material in proving that the defendant committed a particular crime, but it is not essential to a conviction.