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 Montana, as in other jurisdictions, the concept of actus reus requires that a defendant has engaged in a voluntary act, or an omission when there is a legal duty to act, that constitutes the physical element of a crime. This means that for someone to be held criminally responsible, there must be an external component of their behavior that the law recognizes as criminal. The act does not need to be a positive action; it can also be a failure to act when there is a legal obligation to do so. 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 or knowledge that their conduct is criminal. Montana law recognizes various levels of mens rea, from intentional and knowing to negligent, depending on the specific crime. Both actus reus and mens rea are typically required to establish criminal liability, ensuring that both the physical act and the individual's mental state at the time of the act are considered in determining guilt.