An accessory is a person who assists in, but does not actually participate in, the commission of a crime. The distinction between an accessory and a principal is a question of fact and degree:

The principal is the one whose acts or omissions, accompanied by the relevant mens rea (Latin for "guilty mind"), are the most immediate cause of the actus reus (Latin for "guilty act").

If two or more people are directly responsible for the actus reus, they can be charged as joint principals (see common purpose). The test to distinguish a joint principal from an accessory is whether the defendant independently contributed to causing the actus reus rather than merely giving generalized and/or limited help and encouragement.

Elements.

In some jurisdictions, an accessory is distinguished from an accomplice, who normally is present at the crime and participates in some way. An accessory must generally have knowledge that a crime is being committed, will be committed, or has been committed. A person with such knowledge may become an accessory by helping or encouraging the criminal in some way. The assistance to the criminal may be of any type, including emotional or financial assistance as well as physical assistance or concealment.

Relative severity of penalties.

The punishment tariff for accessories varies in different jurisdictions, and has varied at different periods of history. In some times and places accessories have been subject to lesser penalties than principals (the persons who actually commit the crime). In other accessories are considered the same as principals in theory, although in a particular case an accessory may be treated less severely than a principal. In some times and places accessories before the fact (for example, with knowledge of the crime before it is committed) have been treated differently from accessories after the fact (for example, those who aid a principal after a crime has been committed, but had no role in the crime itself). Common law traditionally considers an accessory just as guilty as the principal(s) in a crime, and subject to the same penalties. Separate and lesser punishments exist by statute in many jurisdictions.