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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 California, the legal concept of an accomplice is covered under the state's aiding and abetting laws. According to California Penal Code sections 30-33, an accomplice is someone who assists in the commission of a crime with the intent to facilitate that crime's completion. This assistance can be in the form of encouragement, support, or physical aid. Being an accomplice carries the same legal penalties as if the individual had committed the crime themselves. Furthermore, under California's felony murder rule, an accomplice can be charged with first-degree murder if a death occurs during the commission of certain felonies, such as a bank robbery, even if the accomplice's role was limited to being a lookout or getaway driver. The state does not require the accomplice to have been physically present at the scene of the crime to be charged with the same offense as the principal offender.