A criminal accomplice is a person who knowingly, voluntarily, or intentionally assists another person in the commission of a crime—or under some circumstances, a person who fails to prevent another person from committing a crime. Unlike a person who aids and abets a crime by helping with the planning of the crime but is often not present at the scene of the crime—and unlike an accessory after the fact, who is not present at the scene of the crime but assists after the commission of the crime to help the perpetrator avoid arrest or punishment—an accomplice actively participates in the commission of the crime. For example, a person who acts as a lookout or getaway driver for a bank robbery is an accomplice.

In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a bank robbery that resulted in a death.

In Montana, an individual who assists in the commission of a crime is considered an accomplice and can be charged and punished as if they were the principal offender. Montana law, specifically MCA § 45-2-302, defines an 'accountability for conduct of another' where a person is criminally liable for the conduct of another when acting with the kind of culpability required for the commission of the offense. This includes aiding or abetting the principal offender, or being legally accountable for the behavior of another person. The law does not require the accomplice to be physically present at the scene of the crime to be held liable. Furthermore, under the felony murder rule, an accomplice can be charged with homicide if a death occurs during the commission of a felony, such as a bank robbery, even if the accomplice's role was limited to being a lookout or getaway driver. This is outlined in MCA § 45-5-102, which states that a person commits the offense of deliberate homicide if they purposely or knowingly cause the death of another human being, including situations where a death occurs during the commission of a felony.