In criminal law, mens rea (; Law Latin for "guilty mind") is the mental state of the crime committed and the legal determination of a crime may depend upon both a mental state and actus reus, like the designation of a homicide as murder is a matter of intention to commit a crime or in some jurisdictions knowledge (and reckless disregard) that one's action (or lack of action) would cause a crime to be committed. The mitigation of culpability under some established legal doctrines may reduce the severity of some criminal charges, and so mental state is an element of most crimes, other than crimes of strict liability (like statutory rape where prosecution does not need to prove the defendant knew the age of the victim for a conviction, in some jurisdictions).

The standard common law test of criminal liability is expressed in the Latin phrase actus reus non facit reum nisi mens sit rea, i.e. "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 as a result 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.^: 63–64

The types of mental states that apply to crimes vary depending on whether a jurisdiction follows criminal law under the common law tradition or, within the United States, according to the Model Penal Code.

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In some jurisdictions, the terms mens rea and actus reus have been replaced by alternative terminology.^{: 95 : 84}