Actus reus is the act—or the omission or failure to act when there is a duty to act—that comprises the physical elements of the commission of a crime. The actus reus generally must be voluntary bodily movements that constitute a crime.
In contrast, mens rea (meaning “guilty mind”) is the criminal intent element of a crime. It is the intent to commit a crime, or the knowledge that the action, or failure to act when there is a duty to act, will result in the commission of a crime.

In California, as in other jurisdictions, the concept of actus reus requires a voluntary act, or an omission to act when there's a legal duty to do so, that leads to the commission of a crime. This physical element must be a conscious and voluntary bodily movement. For instance, if someone is coerced into committing a crime under duress, the act may not be considered 'voluntary' in the eyes of the law. On the other hand, mens rea refers to the mental state of the defendant at the time of the crime. It involves an individual's intent to commit a crime or their knowledge that their actions (or inactions, when there is a duty to act) would lead to the commission of a crime. California law recognizes various levels of mens rea, ranging from intentional and willful acts to criminal negligence. Both actus reus and mens rea must be established for a defendant to be found guilty of most crimes, although there are exceptions, such as strict liability offenses, where mens rea is not a required element.