Mens rea (Law Latin for "guilty mind") is the mental element of a person's intention to commit a crime; or knowledge that one's action or lack of action would cause a crime to be committed. It is a necessary element of many crimes.

The standard common law test of criminal liability is expressed in the Latin phrase actus reus non facit reum nisi mens sit rea, for example, "the act is not culpable unless the mind is guilty". As a general rule, someone who acted without mental fault is not liable in criminal law. Exceptions are known as strict liability crimes. Moreover, when a person intends a harm, but because of bad aim or other cause, the intent is transferred from an intended victim to an unintended victim, the case is considered to be a matter of transferred intent.:

In civil law, it is usually not necessary to prove a subjective mental element to establish liability for breach of contract or tort, for example. But if a tort is intentionally committed or a contract is intentionally breached, such intent may increase the scope of liability and the damages payable to the plaintiff.

In some jurisdictions, the terms mens rea and actus reus have been replaced by alternative terminology.

Levels of mens rea.

Under the traditional common law, the guilt or innocence of a person relied upon whether he had committed the crime (actus reus), and whether he intended to commit the crime (mens rea). However, many modern penal codes have created levels of mens rea called modes of culpability, which depend on the surrounding elements of the crime: the conduct, the circumstances, and the result, or what the Model Penal Code calls CAR (conduct, attendant circumstances, result). The definition of a crime is thus constructed using only these elements rather than the colorful language of mens rea:

Murder is the unlawful killing of a human being with malice aforethought.

— 18 U.S.C. § 1111 (traditional common law).

A person commits an offense if he: (1) intentionally or knowingly causes the death of an individual.

— portion of Texas Penal Code § 19·02 (modern offense element).

The traditional common law definitions and the modern definitions approach the crime from different angles.

In the common law approach, the definition includes:

1. actus reus: unlawful killing of a human being.

2. mens rea: malice aforethought.

Modern criminal law approaches the analysis somewhat differently. Using a framework from the American Law Institute's Model Penal Code, homicide is a "results" offense in that it forbids any "purposeful" or "knowing" conduct that causes, and therefore results in the death of another human being. "Purposeful" in this sense means the actor possessed a conscious purpose or objective that the result (for example, the death of another human being) be achieved. "Knowing" means that the actor was aware or practically certain that a death would result but had no purpose or desire that it occur. Many states still adhere to older terminology, relying on the terms "intentional" to cover both types of mens rea: "purposeful" and "knowing."

Thus, the actus reus and mens rea of homicide in a modern criminal statute can be considered as follows:

1. actus reus: any conduct resulting in the death of another individual.

2. mens rea: purposeful intent or knowledge that the conduct would result in the death.

In the modern approach, attendant circumstances sometimes replace traditional concepts of mens rea, indicating the level of culpability as well as other circumstances. For example, the crime of theft of government property would include as an attendant circumstance that the property belong to the government, instead of requiring that the accused have actual awareness that the property belongs to the government