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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 Massachusetts, the law does not strictly differentiate between principals and accomplices in the commission of a crime. Both parties can be charged and punished equally under the state's joint venture doctrine. This means that a person who assists, aids, or participates in a crime in any way can be held as responsible as the person who actually commits the criminal act. For instance, a lookout or getaway driver in a bank robbery can face the same charges as the individual who physically robs the bank. Additionally, under Massachusetts law, the felony murder rule applies, which can result in an accomplice being charged with first-degree murder 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 driver. This reflects the state's approach to holding all participants in a criminal enterprise accountable for the outcomes of the criminal acts they support or enable.