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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 Illinois, the concept of a criminal accomplice is covered under the state's complicity laws. An individual is considered an accomplice if they knowingly and intentionally assist in the commission of a crime. This can include acts before, during, or after the crime that contribute to its execution or to the avoidance of detection or arrest. Illinois law does not strictly differentiate between principals and accomplices; rather, it holds all parties accountable for the crime itself under the theory of accountability. According to 720 ILCS 5/5-2, a person is legally accountable for the conduct of another when they solicit, aid, abet, agree, or attempt to aid the other person in the planning or commission of the offense. Furthermore, under the felony murder rule in Illinois, an accomplice can be charged with first-degree murder if a death occurs during the commission of a forcible felony, such as a bank robbery, even if the accomplice's role was limited to being a lookout or getaway driver. This reflects the state's stance on holding all participants in a crime responsible for the outcomes of the criminal act.