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Criminal law basics

From criminallaw.wiki, the sourced reference on US criminal law
General information, not legal advice. Criminal laws differ by state and change often. For advice about a specific situation, talk to a licensed criminal defense lawyer or a public defender. See the legal disclaimer.

Law checked as of · Written by the criminallaw.wiki editorial team

Key facts: A crime is any act or omission that violates a law prohibiting it, and the government must prove every element of the offense beyond a reasonable doubt.[1]

Criminal law is the body of rules under which the government, rather than a private party, decides whether to punish someone for an act or omission.[1]It differs from civil law, which resolves disputes between private parties over their rights.[1]Every U.S. jurisdiction defines its own crimes, but most criminal codes share the same basic building blocks and grading system.

What criminal law is

Elements of a crimeActus reus(guilty act or omission)Mens rea(guilty mind)Concurrence(act and intent together)Causation(act causes the harm)Crime provenbeyond a reasonable doubt
The building blocks most crimes share. The prosecution must prove every element beyond a reasonable doubt; strict liability offenses drop the mental-state element. Diagram: criminallaw.wiki.[5][6][7]

Criminal law is a system of laws concerned with crimes and the punishment of individuals who commit them, distinguished from civil law, where two private parties dispute their rights.[1]A criminal prosecution involves the government deciding whether to punish an individual for an act or an omission, and a crime is simply any act or omission that violates a law prohibiting it.[1]For a fuller discussion of this distinction, see what is criminal law? and criminal law vs civil law.

The building blocks of a crime

An individual commits a crime only when their conduct fulfills every element of an offense, and the statute defining the crime also defines its elements.[1]In general, every crime involves three elements: the act or conduct, known as actus reus; the person's mental state at the time of the act, known as mens rea; and causation linking the act to the result.[1]The government carries the burden of proving every element beyond a reasonable doubt.[1]

Actus reus must be a voluntary affirmative act, or in some cases an omission where the person had a legal duty to act, such as a duty created by statute, contract, a special relationship, or a voluntarily assumed duty of care.[5]An involuntary act, such as a reflex, does not satisfy this requirement.[5]Mens rea is the guilty mind requirement, and most states organize mental states hierarchically using the Model Penal Code's four categories: acting purposely, knowingly, recklessly, or negligently, with a fifth category of strict liability where no guilty mind need be shown at all.[6]Where a statute does not specify a mental state, courts will still require the government to prove only the mens rea necessary to separate wrongful from innocent conduct.[6]More detail on these concepts appears at elements of a crime and mens rea (the guilty mind).

How crimes are graded

Crimes are generally separated into four categories: felonies, misdemeanors, inchoate offenses, and strict liability offenses, with each state and the federal government deciding what conduct falls into each category.[1]A felony is typically graded by the maximum term of imprisonment authorized, ranging from Class A felonies carrying life imprisonment or death down to Class E felonies carrying more than one but less than five years, though some states grade felonies crime-by-crime instead.[2]A misdemeanor is typically punishable by less than twelve months in jail, with common punishments including fines, probation, community service, or short jail terms, and misdemeanors are similarly sometimes broken into letter-graded classes based on the maximum term authorized.[3]For example, under one state's penal law, felonies are divided into nine levels ranging from a lower-degree fraud offense up to terrorism.[1]See felony vs misdemeanor vs infraction for more on this distinction, and criminal sentencing for how grading affects punishment.

Where criminal law comes from

Each state decides what conduct to designate as a crime, so each state has its own criminal code, and Congress has also codified federal criminal law, primarily in Title 18 of the U.S. Code.[1]Criminal laws vary significantly among the states and the federal government; some state statutes resemble older common law crime definitions, while others closely follow the Model Penal Code.[1]The Model Penal Code was assembled by the American Law Institute and first issued in 1962, and many states' criminal codes have since been reformed based on it, particularly its general principles of liability and its definitions of specific offenses.[4]The MPC also standardized the mens rea terms used to describe a defendant's mental state.[4]Congress has only limited power to create criminal laws compared with the states, which is why state codes tend to be far more extensive than the federal code.[1]The federal government has also codified criminal procedure separately in the Federal Rules of Criminal Procedure.[1]For an overview of specific offense categories, see crimes and offenses, and for how a federal or state case actually proceeds, see the criminal process step by step.

Entries in this section

Further entries planned for this section (not yet published):

Common questions

What are the building blocks every crime shares?

Most crimes require proof of an act or omission (actus reus), a mental state (mens rea), and causation connecting the act to a prohibited result.[1]The actus reus must generally be a voluntary act, or an omission where the person had a legal duty to act.[5]The mens rea requirement means the defendant must have acted with a culpable state of mind, usually categorized as purposeful, knowing, reckless, or negligent conduct.[6]

How are crimes classified?

Crimes are generally divided into felonies, misdemeanors, inchoate offenses, and strict liability offenses.[1]Felonies are the more serious category, with classes ranging up to life imprisonment or death, while misdemeanors are typically punishable by less than twelve months in jail.[2][3]Not every state uses letter-graded classes; some grade sentencing crime-by-crime instead.[2][3]

Where does criminal law come from?

Each state writes its own criminal code, and the federal government has codified its own crimes mainly in Title 18 of the U.S. Code.[1]Many states have reformed their codes based on the Model Penal Code, a model statute drafted by the American Law Institute.[4]Because the power to criminalize conduct is largely reserved to the states, state codes are typically far more extensive than federal law.[1]

Can someone be punished just for their status rather than an act?

No; the Supreme Court has held that criminalizing a person's status, rather than their conduct, inflicts cruel and unusual punishment in violation of the Eighth and Fourteenth Amendments.[1]For example, a state cannot punish someone simply for being homeless, though it can punish conduct like trespassing or loitering.[1]

Does a defendant need to know their conduct is illegal to be guilty?

No; the mens rea requirement means a defendant must be aware of the facts that make the conduct fit the definition of the offense, but the defendant need not know that the conduct is illegal.[6]Some offenses, called strict liability crimes, do not require any guilty mental state at all, such as certain possession offenses.[6]

References

All sources retrieved November 2026.

  1. ^ a b c d e f g h i j k l m n o p q r s t u v w x "Criminal law (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  2. ^ a b c d "Felony (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  3. ^ a b c d "Misdemeanor (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  4. ^ a b c d "Model Penal Code (MPC) (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  5. ^ a b c d "Actus reus (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  6. ^ a b c d e f "Mens rea (Wex)". Legal Information Institute, Cornell Law School. Retrieved November 2026.
  7. ^ "In re Winship, 397 U.S. 358 (1970)". U.S. Supreme Court opinion via Legal Information Institute. Retrieved November 2026.