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In many states the traditional distinctions between the culpability of accomplices and principals to a crime have been replaced by statute—including the felony murder rule that may make an accomplice guilty of first degree or capital murder if he was the lookout or getaway driver for a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 New Hampshire, as in many states, the law does not make a sharp distinction between the principal offender and an accomplice in the commission of a crime. Under New Hampshire Revised Statutes Annotated (RSA) 626:8, a person is considered an accomplice to an offense if they knowingly aid or agree to aid, or attempt to aid another person in planning or committing the offense. The accomplice's liability extends to the conduct of the person they assist, provided the accomplice acts with the intent to promote or facilitate the commission of the offense. This means that an accomplice can face the same charges and penalties as the principal offender. Furthermore, under the felony murder rule in New Hampshire (RSA 630:1-b), if a death occurs during the commission of certain felonies, such as a bank robbery, all participants, including accomplices like lookouts or getaway drivers, can be charged with first-degree murder, regardless of their direct involvement in the actual killing.