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Criminal Law

Categories Of Criminal Conduct, Elements Of A Crime: Mens Rea And Actus Reus, Defenses

Criminal law is comprised of rules and statutes intended to dictate parameters of conduct that will prevent harm to society. Criminal law differs from criminal procedure. Criminal law is concerned with defining crimes and setting punishment, while criminal procedure refers to the process by which those laws are enforced. For example, substantive criminal law is used to determine whether someone has committed arson. Criminal procedure guides the prosecution of the arsonist, from evidence gathering to trial and beyond. Criminal procedure encompasses constitutional protections such as the right to be free from unreasonable searches and seizures, the right to counsel, the right against self-incrimination, and the right to trial by jury.

In England criminal law arose from case law, rather than from codified laws. Known as part of the common law, these rules formed the foundation of U.S. criminal laws. Some crimes are still defined by their common law meanings, but much of this common law has been codified in statutes. Other crimes, such as embezzlement and receiving stolen property, are wholly statutory creations.

The Necessary and Proper Clause of the Constitution grants Congress the authority to denominate certain conduct illegal. States may make laws prohibiting and punishing certain acts, so long as the law does not contravene the U.S. Constitution or state constitutions, and so long as the conduct prohibited is reasonably related to protecting the welfare and safety of society. Municipalities may also designate illegal behavior, based upon limited powers delegated to them by the state legislature.


A person may use the amount of force reasonably necessary to prevent immediate unlawful imposition of harm to oneself. In addition, a defendant generally must not be the provoker. Deadly force may be used to repel someone when it reasonably appears necessary to prevent imminent death or serious injury. Some jurisdictions require a defendant to retreat before using deadly force, if it can be done safely.

Further Readings

  • Loewy, Arnold H. Criminal Law in a Nutshell. St. Paul, MN: West Publishing Co. 1987.
  • West's Encyclopedia of American Law (various sections). St. Paul, MN: West Publishing Co. 1997.

Additional topics

Law Library - American Law and Legal InformationGreat American Court Cases