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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 Washington State, the law does not distinguish sharply between principals and accomplices in the commission of a crime. Under Revised Code of Washington (RCW) 9A.08.020, a person is considered an accomplice to a crime if they, with knowledge that it will promote or facilitate the commission of the crime, (1) solicit, command, encourage, or request another person to commit the crime; or (2) aid or agree to aid another person in planning or committing the crime. The statute applies regardless of the presence of the accomplice at the actual scene of the crime. Furthermore, Washington's felony murder rule, as outlined in RCW 9A.32.030, can hold an accomplice liable for first-degree murder if a death occurs during the commission or attempted commission of certain felonies, such as robbery, even if the accomplice's role was limited to being a lookout or getaway driver. This means that accomplices in Washington can face the same degree of criminal liability as the principal offender.