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 South Dakota, as in other jurisdictions, the concept of actus reus requires that a defendant must have 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 an individual to be held liable for a criminal offense, there must be evidence of their actual conduct that breached the law. This could include any action, conduct, or behavior that is prohibited by law. On the other hand, mens rea refers to the mental state of the defendant at the time of the crime. South Dakota law recognizes that for most crimes, there must be a certain level of intent or knowledge on the part of the defendant. This intent or knowledge is what constitutes the mens rea, or 'guilty mind,' and it must be proven alongside the actus reus for a conviction to occur. The specific requirements for mens rea can vary depending on the crime, but it generally involves proving that the defendant acted with purpose, knowledge, recklessness, or negligence, as defined under South Dakota's criminal statutes.