Criminal Law


  1. Overview
  2. Elements of a Crime
  3. Types of Crimes
  4. Liability for Accomplices
  5. Defenses
  6. Additional Sources


Criminal Law, as distinguished from civil law, is a system of laws concerned with punishment of individuals who commit crimes. Thus, where in a civil case two individuals dispute their rights, a criminal prosecution involves the people as a whole deciding whether to punish an individual for his conduct or lack of conduct (i.e. omission). Just as the people decide what conduct to punish, so the people decide what punishment is appropriate. Accordingly, punishments vary with the severity of the offense—from a simple fine (e.g. for a traffic violation) to loss of freedom (e.g. for murder).

Each state decides what conduct to designate a crime. Thus, each state has its own criminal code. Congress has also chosen to punish certain conduct, codifying federal criminal law in Title 18 of the U.S. Code. Criminal laws vary significantly among the states and the Federal Government. While some statutes resemble the common law criminal code, others, like the New York Penal Law, closely mimic the Model Penal Code (MPC).

A “crime” is any act or omission in violation of a law prohibiting it, or omitted in violation of a law ordering it. The government cannot prosecute an individual for conduct that was not declared criminal at the time the individual acted. The Constitution explicitly forbids in Article 1, Sections 9 and 10 retroactively applicable criminal laws—ex post facto laws.

It is also important to note that a law cannot punish a person simply for their status. As the Supreme Court explained in Robinson v. California, 370 U.S. 660 (1962), any statute that criminalizes the status of a person inflicts a cruel and unusual punishment in violation of the Eight and Fourteenth Amendments. For example, a state could not punish an individual for “being homeless,” which would be a status offense, but could punish a homeless individual for trespassing or loitering, which involves some conduct.

In general, Criminal Law asks and answers three questions:

  1. Did an individual commit a crime?
  2. Which crime did an individual commit?
  3. Does the individual have a defense?

This page will briefly explore how Criminal Law deals with these questions and suggest additional resources for a more thorough study.


Elements of a Crime

An individual commits a crime if he or she acts in a way that fulfills every element of an offense. The statute establishing the offense also establishes the elements of the offense. In general, every crime involves three elements: first, the act or conduct (“actus reus”); second, the individual’s mental state at the time of the act (“mens rea”); and third, the causal link between the act and the offense. In a criminal prosecution, the government has the burden of proof to establish every element of a crime beyond a reasonable doubt; and third, the individual's conduct must be the cause of the crime.

The actus reus is the voluntary act that either is in itself wrongful or leads to a wrongful result. For example, driving while drunk is an act that is in itself wrongful. Conduct that causes the death of another, on the other hand, is only wrongful if it leads to a specific result—the death of another. Thus, some laws only punish conduct (i.e. conduct crimes), while others punish the result (i.e. result crimes). In every case, an individual is only guilty of a crime if he or she voluntarily acted. This means that an individual is not guilty if he or she acted on reflex, under hypnosis, or due to a convulsion. In certain circumstances, however, an individual could be found guilty for not acting at all (i.e. failing to act). In other words omission can serve to establish the actus reus if an individual was under a duty to act and failed to discharge that duty.

Regardless of whether an individual is being prosecuted for an affirmative act or an omission, an individual is only guilty if he or she had the requisite mental state ("mens rea") when engaging in an act or an omission. The statute codifying the crime typically prescribes the mental state requisite for a prosecution. For example:

  • If a statute defined burglary as breaking into the dwelling house of another with intent to commit felony therein, an individual could only be found guilty of burglary if the prosecution could establish that the individual intended to commit a felony.
  • If a statute defined murder as knowingly causing the death of another, an individual could only be found guilty of murder if he or she knew that his or her conduct would cause the death of another.
  • If a statute defined involuntary manslaughter as negligently causing the death of another, an individual could only be found guilty of involuntary manslaughter if he or she caused the death of another by failing to exercise that level of care that a reasonable person would.

When a statute does not prescribe a specific mental state, a court will read in “that mens rea which is necessary to separate wrongful from innocent conduct.”

Even if an individual acted with an unlawful mental state, an individual cannot be convicted if he caused no crime. Of course, if the act itself is criminal, then the "cause" requirement is satisfied the moment the individual acts. When a statute punishes the result of some conduct, however, the prosecution must prove that the conduct was the actual cause and the legal cause of the unlawful result. In other words, the prosecution must prove that the conduct was the but-for and proximate cause of the result. 

For additional study of actus reus and mens rea consult the following resources:


Types of Crimes

Crimes can be generally separated into four categories: felonies, misdemeanors, inchoate offenses, and strict liability offenses. Felonies are typically punished by more than a year in prison, while misdemeanors are punished by less than a year. For strict liability offenses, all the prosecution needs to prove is that an individual engaged in certain conduct. Once the conduct, the actus reus, requirement has been satisfied, the individual is found guilty regardless of whether the individual knew or even should have known that he or she was engaging in forbidden conduct. For example, statutory rape is a strict liability offense.

Each state, and the federal government, decides what sort of conduct to criminalize. At common law, there were eight major felonies (e.g. Murder, Robbery, Manslaughter, Rape, Sodomy, Larceny, Arson, Mayhem, and Burglary) and various misdemeanors (e.g. assault, battery, false imprisonment, perjury, and intimidation of jurors).

Congress codified the federal criminal law and criminal procedure in Title 18 of the U.S. Code with §§ 1 to 2725 dealing with crimes. Title 18 designates various conduct as federal crimes, such as arson, use of chemical weapons, counterfeit and forgery, embezzlement, espionage, genocide, and kidnapping. These statutes usually prescribe a maximum sentence appropriate for a convicted individual. For additional Federal Regulations, consult 28 C.F.R.

The U.S. Code is far more extensive than the common law. Nonetheless, Congress has limited power to make criminal laws. As this power is generally reserved to the states, state criminal codes, such as the New York Penal Law, are far more complicated than the U.S. Code. The N.Y. Penal Law prescribes nine levels of felonies, ranging from residential mortgage fraud in the fourth degree to the crime of terrorism. New York legislature has also passed sentencing guidelines—a range of months or years a person should be sentenced for following a conviction. For example, a person convicted of the crime of terrorism, as a class ‘A’ violent felony, faces anywhere between a life sentence and a twenty- to twenty-five-year sentence. A person who is convicted of residential mortgage fraud in the fourth degree, as a class ‘E’ nonviolent felony, faces a probation of sixteen to forty-eight months.

For criminal law statutes for states other than New York, consult Criminal Code – By State.

For further study, consult the following sources:


Liability for Accomplices

When multiple parties are involved, the traditional first step is to classify the participants according to the following categories:

  1. Principal in the first degree – those who actually commit a crime (i.e. the perpetrator). Perpetrators are not accomplices and this section does not pertain to them.
  2. Principal in the second degree – those who aided, counseled, commanded, or encouraged the perpetrator in the actual commission of a crime. An abettor is considered an accomplice.
  3. Accessory before the fact – those who aided, counseled, commanded, or encouraged the perpetrator to commit the crime, without actually being present at the moment of perpetration. An accessory (before the fact) is considered an accomplice.
  4. Accessory after the fact – those who aid an individual, knowing him or her to be a criminal, in an effort to hinder the individual’s detection, arrest, trial, or punishment. Accessories (after the fact) are guilty of a separate crime, so this section does not pertain to them.

To convict an accomplice, the prosecution needs to establish the requisite actus reus and mens rea. That is, the prosecution must prove that the accomplice acted in support of the perpetrator, and had the requisite mental state while doing so. It is important to note that some jurisdictions allow accomplices to be prosecuted independently of the principal perpetrator. Thus, an accomplice could be found guilty of more odious offense than the principal. In fact, an accomplice could be convicted while the alleged perpetrator is acquitted.

For additional study of accomplice liability, consult the following sources:



There are a number of defenses available to a defendant in a criminal prosecution. The following list illustrates some common defenses individuals rely on:

  • Failure of Proof – an individual’s simplest defense in a criminal prosecution is to claim that the prosecution has not or cannot prove an element of the offense.
  • Mistakes – in certain circumstances, an individual’s mistake can be used as a defense.|
    • Mistake of Law – a mistake regarding the legal status or effect of some situation.
    • Mistake of Fact – a mistake regarding the facts of some situation.
  • Justifications – defenses that, if applicable, justify an individual’s conduct (i.e. you did not do anything wrong).
    • Self-Defense: the use of force to protect oneself from an attempted injury by another.
    • Defense of property: a person may use force to protect his property from a felony occurring within.
    • Defense of Others: is the right of a person to protect a third party with reasonable force against an assailant who seeks to inflict force upon the third party.
    • Necessity: sometimes referred to as the “choice of evils,” the necessity defense allows an individual to engage in otherwise unlawful conduct if by doing so the individual avoids a greater harm.
  • Excuses – defenses that, if applicable, excuse misconduct (i.e. you did something wrong, but society does not judge you for it).
    • Duress: an individual may plead duress if another individual forced him or her to engage in the illegal conduct by force or threat of force.
    • Intoxication: an individual who was involuntarily intoxicated can plead intoxication as a defense to every crime. An individual who was voluntarily intoxicated can plead intoxication as a defense only to crimes that require a specific mental state. 
    • Insanity: an insane individual cannot form the requisite mental state, and thus cannot be found guilty.


Additional Sources


Last updated in June of 2016 by Eugene Temchenko