Punishment is justice for the unjust.
—Augustine （British writer）
刑法是国家为了维护统治秩序，规定什么是犯罪及对犯罪行为处以何种刑罚（即相对应法律效果）的法律。与民法等其他法律相比，刑法的特点是处罚的手段十分严厉，强制程度高，不仅能限制人身自由，甚至可以剥夺人的生命。刑法的目的在于界定国家对于公民的刑罚权，刑罚的目的通常是对侵害法律权益的行为加以处罚，借由处罚吓阻犯罪人及社会大众，教化犯罪人使其社会化。国家就是运用这种强大的威慑力量，同各种破坏社会秩序的犯罪作斗争。
在美国，各州法律中关于犯罪定义和种类的规定并不尽相同。一般来说，犯罪可以分为重罪（ felony）和轻罪（ misdemeanor）两大类。而每一种犯罪又可以分为若干等级，以表明其相对的严重性。刑罚一般都由法官确定。至今没有制定统一的刑法典，只是在1962年公布了一部《标准刑法典草案》，供各州立法参考，但是联邦和各州多有指导法官判刑的量刑指南（ Guidelines for Sentencing）。美国1965年废除了死刑（ capital punishment） ，但是在1976年又开始恢复死刑。南部各州恢复死刑较早。纽约州直到1995年才恢复死刑。目前，美国大约3/4的州的法律规定有死刑，而没有死刑的州主要集中在美国的东部和北部。美国的死刑上诉程序十分复杂，而且名目繁多，耗费时间，因此，死囚犯在狱中等待死刑的时间超过20年者屡见不鲜。有趣的是，美国监禁刑期没有最高限，在数罪并罚时可高达一二百年。
Objectives of Criminal Law
The term criminal law， sometimes called penal law， refers to any of various bodies of rules in different jurisdictions whose common characteristic is the potential for unique and often severe impositions as punishment for failure to comply. Criminal punishment， depending on the offense and jurisdiction， may include execution， loss of liberty， government supervision（ parole or probation） ， or fines. Criminal law typically is enforced by the government， unlike the civil law， which may be enforced by private parties.
Criminal law is distinctive for the uniquely serious potential consequences or for failure to abide by its rules. Five objectives are widely accepted for enforcement of the criminal law by punishments： retribution， deterrence， incapacitation， rehabilitation and restitution. Jurisdictions differ on the value to be placed on each.
Retribution—Criminals ought to suffer in some way. This is the most widely seen goal. Criminals have taken improper advantage， or inflicted unfair detriment upon others and consequently， the criminal law will put criminals at some unpleasant disadvantage to“balance the scales”. If people contravene these laws， they surrender the rights granted to them by the law. Thus， one who murders may be murdered himself.
Deterrence—Individual deterrence is aimed toward the specific offender. The aim is to impose a sufficient penalty to discourage the offender from criminal behavior. General deterrence aims at society at large. By imposing a penalty on those who commit offenses， other individuals are discouraged from committing those offenses.
Incapacitation—Designed simply to keep criminals away from society so that the public is protected from their misconduct.This is often achieved through prison sentences today. The death penalty or banishment have served the same purpose.
Rehabilitation—Aims at transforming an offender into a valuable member of society. Its primary goal is to prevent further offense by convincing the offender thattheir conduct was wrong.
Restitution—This is a victim-oriented theory of punishment. The goal is to repair， through state authority， any hurt inflicted on the victim by the offender. For example， one who embezzles will be required to repay the amount improperly acquired. Restitution is commonly combined with other main goals of criminal justice and is closely related to concepts in the civil law.
Many laws are enforced by threat of criminal punishment， and their particulars may vary widely from place to place. The entire universe of criminal law is too vast to intelligently catalog. Nevertheless， the following are some of the more known aspects of the criminal law.
The criminal law generally prohibits undesirable acts. Thus， proof of a crime requires proof of some act. Scholars label this the requirement of an actus reus or guilty act. Because of the potentially severe consequences of criminal conviction， judges at common law also sought proof of an intent to do some bad thing， the mens rea or guilty mind. As to crimes of which both actus reus and mens rea are requirements， judges have concluded thatthe elements must be present at precisely the same moment and it is not enough that they occurred sequentially at different times.
Actus reus
Actus reus is Latin for“guilty act”and is the physical element of committing a crime. It may be accomplished by an action， by threat of action， or exceptionally， by an omission to act. For example， the act of A striking B might suffice， or a parent s failure to give food to a young child also may provide the actus reus for a crime.
Where the actus reus is a failure to act， there must be a duty of care. A duty can arise through contract， a voluntary undertaking， a blood relation with whom one lives， and occasionally through one s official position. Duty also can arise from one s own creation of a dangerous situation. Occasional sources of duties for bystanders to accidents in Europe and North America are good Samaritan laws， which can criminalise failure to help someone in distress （ e.g. a drowning child） . In this case it was held that since a cancer patient could not give or withhold consent to medical treatment， it was for the doctors to decide whether treatment was in the patients best interest. It was reasonable for them to conclude that treatment was not in the patients best interest， and should therefore be stopped， when there was no prospect of improvement. It was never lawful to take active steps to cause or accelerate death，although in certain circumstances it was lawful to withhold life sustaining treatment， including feeding， without which the patient would die.
An actus reus may be nullified by an absence of causation. For example， if a crime involves harm to a person， the person s action must be the but for cause and proximate cause of the harm. Causation is not broken simply because a victim is particularly vulnerable. However， it may be broken by an intervening act of a third party， the victim s own conduct， or another unpredictable event.