A body of rules, case law and statutes that defines conduct prohibited by the government because it threatens or harms public safety and welfare and that establishes punishment to be imposed for the commission of such acts.

Substantive criminal laws define crimes and establishes punishments for those crimes. In contrast, criminal procedure describes the process through which the criminal laws are enforced. For example, the law prohibiting murder is a substantive criminal law. The manner in which government enforces this substantive law-through the gathering of evidence and prosecution-is generally considered a procedural matter. Crimes are usually categorized as felonies or misdemeanors based on their nature and the maximum punishment that can be imposed. A felony involves serious misconduct that is punishable by death or by imprisonment for more than one year. Most state criminal laws classify felonies with varying degrees of punishment. Crimes that do not amount to felonies are misdemeanors.

A misdemeanor is misconduct for which the law prescribes punishment of no more than one year in jail. Lesser offenses, such as traffic infractions, are often called violations and are generally considered a part of criminal law. The power to make certain conduct illegal is granted to Congress by virtue of the Necessary and Proper Clause of the Constitution (art. I, ยง 8, cl. 18). Congress has the power to define and punish crimes whenever it is necessary and proper to do so, in order to accomplish and safeguard the goals of government and of society in general. Congress has wide discretion in classifying crimes as felonies or misdemeanors, and it may revise the classification of crimes. State legislatures have the exclusive and inherent power to pass laws prohibiting and punishing any act, provided that the law does not contravene the provisions of the U.S. or state constitution. When classifying conduct as criminal, state legislatures must ensure that the classification bears some reasonable relation to the welfare and safety of society. Municipalities may make designated behavior illegal insofar as the power to do so has been delegated to them by the state legislature. Laws passed by Congress or a state must define crimes with certainty.