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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 Maine, an individual who assists in the commission of a crime may be charged as an accomplice. Maine law does not strictly differentiate between principals and accomplices; both can be held liable for the crime itself. Under Maine's criminal code, a person is an accomplice if they intentionally aid or attempt to aid another person in planning or committing a crime. This includes acting as a lookout or getaway driver during the commission of a crime such as a bank robbery. Furthermore, Maine follows the felony murder rule, which means that if a death occurs during the commission of a felony, such as a bank robbery, an accomplice can be charged with felony murder, potentially leading to first-degree murder charges. This holds true even if the accomplice's role was limited to assistance and they were not the direct perpetrator of the fatal act.