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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 Idaho, the legal concept of an accomplice is covered under the state's criminal aiding and abetting statutes. According to Idaho Code § 18-204, an accomplice is someone who, with the intent to promote or assist the commission of a crime, aids, abets, advises, hires, counsels, or encourages another person in planning or committing the crime. The law does not require the accomplice to be physically present at the crime scene to be held liable. Furthermore, under Idaho's felony murder rule, as per Idaho Code § 18-4003, if a person is an accomplice in the commission of a felony such as a bank robbery, and a death occurs during the commission of that felony, the accomplice can be charged with first-degree murder, regardless of whether they were the direct cause of the death. This reflects the state's approach to holding accomplices to the same degree of responsibility as the principal offender in serious crimes.