What Is Criminal Law?

Criminal law is a complex system of laws and rules that define criminal acts, set punishments, and outline the rules guiding the process from investigation and arrest to sentencing and parole.

By , Attorney · Mitchell Hamline School of Law

The area of criminal law primarily concerns those accused of, or convicted for, committing a crime. Criminal law is a complex system of laws (typically called statutes and ordinances) and procedures (such as rules of court procedure and evidence) that define criminal acts, set punishments, and outline the rules guiding the criminal process from investigation and arrest to sentencing and parole.

The U.S. Constitution and each state's constitution set limits on what the government can and can't do when it comes to criminal laws and procedures. Courts interpret how statutes and rules of procedure are applied in a criminal case, as well as if these laws violate constitutional limits. Court decisions (case law) provide further guidance on the law.

What Is a Crime?

A crime is wrongful conduct which is prohibited by law and may be punished by a loss of liberty (incarceration). State and federal lawmakers define crimes and their punishments in statute.

Criminal vs. Civil Law

Crimes generally represent conduct that causes a public harm to society as a whole and goes beyond injuries to private parties. For instance, a breach of contract primarily affects the parties to the contract (and is a civil action), but criminal acts—like murder, impaired driving, or theft—cause injury and harm to individual victims as well as to society.

Criminal law differs from civil law in other respects, as well. For instance, a government lawyer (called a prosecutor) brings criminal charges against the accused, usually on behalf of the state or federal government. In contrast, a private lawyer files a civil lawsuit to resolve a dispute between private parties. Criminal charges or a conviction can result in imprisonment and fines, whereas a civil lawsuit typically results in payment of monetary damages or changes to a party's legal status (such as divorce or parenting rights) but not imprisonment.


Typically, the greater the public or social harm involved, the greater a crime's potential punishment will be. The law generally punishes crimes by their severity in terms of harm and blameworthiness. In other words: "Let the punishment fit the crime." (W.S. Gilbert). The penalty imposed for committing a crime represents not only punishment for an offender but also is meant to deter (or discourage) someone from committing a crime in the first place.

How Are Crimes Classified?

Crimes are classified by their severity into two main categories: felonies and misdemeanors. A third category, infractions, often involves the criminal process but is a fine-only offense.


A felony can typically be punished by more than a year in prison. Felonies often involve offenses that can or do result in serious physical harm (such as murder or assault with a deadly weapon) but also include offenses that involve serious societal harm (such as mortgage fraud and bribing public officials).


Misdemeanors are less serious crimes that generally carry a punishment of up to a year in jail. Examples of misdemeanors include petty theft, vandalism, and careless driving. In certain cases, misdemeanors can be elevated to felonies—for instance, repeat misdemeanor assaults occurring within a certain amount of time could be classified as a felony under statute.


An infraction includes petty offenses such as traffic violations. Because infractions can result only in fines and not imprisonment, they are not technically crimes. However, often an infraction involves some interaction with the criminal process (such as being pulled over by the police during a traffic stop), and certain criminal laws and procedures come into play.

What Are Criminal Rules of Procedure?

Criminal rules of procedure and evidence provide the playbooks for the criminal process. For instance, they dictate:

  • when warrants may be issued
  • how pretrial, trial, and post-trial hearings are conducted, and
  • what evidence is or isn't admissible at trial.

The rules also set timelines for court proceedings, such as bail hearings. The judicial branch of a jurisdiction writes and amends court rules.

What Are the Constitutional Limits in Criminal Law?

The U.S. Constitution grants people accused of crimes certain rights to protect them from being treated unfairly. Some of these rights include:

  • the right to an attorney
  • the right to not incriminate oneself
  • the right to a speedy and public trial, and
  • the right to an appeal.

Each state also has a constitution that may provide its citizens with greater rights than those given under the U.S. Constitution.

What Is Case Law?

The judicial branch interprets how statutes and rules of procedure are applied in a criminal case, as well as if these laws violate constitutional limits. If a defendant or the government believes a trial judge incorrectly applied the law in a case or a statute violates a constitutional limit, they may challenge the ruling or law. Such challenges may be brought at various stages of the criminal process (not just after a conviction). An appeals court's (published) ruling on the challenge becomes the law in that jurisdiction.

For instance, you might have heard of the Miranda warning—the warning an officer must give suspects before a custodial interrogation to inform suspects of their constitutional rights to remain silent and consult an attorney. The Miranda warning was named for a case brought by Ernesto Miranda who confessed to a crime after being interrogated for two hours. The U.S. Supreme Court held that his confession should have been tossed out because the police did not advise Mr. Miranda of his rights to remain silent and consult an attorney. The Miranda decision is now the law of the land. (Miranda v. Arizona, 384 U.S. 436 (1966).)

Should I Speak to an Attorney If Charged With a Crime?

Yes. If you've been accused of committing a crime, you should speak to a criminal defense attorney who can help you understand and protect your rights and develop a strong defense for your case. An attorney can also explain any immediate or future consequences of decisions you make, such as talking to the police, pleading guilty, and testifying at trial.

Consequences for those charged with, or convicted of, a crime can change a person's life forever. And these consequences extend far beyond arrest, trial, incarceration, and supervision. A criminal record can affect employment and educational opportunities, family relationships and stability, personal liberties, and more.

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