An accessory is a person who assists, 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:

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.

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 others 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 (i.e., with knowledge of the crime before it is committed) have been treated differently from accessories after the fact (e.g., 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.

In some situations, a charge of conspiracy can be made even if the primary offense is never committed, so long as the plan has been made, and at least one overt act towards the crime has been committed by at least one of the conspirators. For example, if a group plans on forging bank checks, and forges the checks but ultimately does not attempt to cash the checks, the group might still be charged with conspiracy due to the overt act of forgery. Thus, an accessory before the fact will often, but not always, also be considered a conspirator. A conspirator must have been a party to the planning of the crime, rather than merely becoming aware of the plan to commit it and then helping in some way.

A person who incites another to a crime will become a part of a conspiracy if agreement is reached and may then be considered an accessory or a joint principal if the crime is eventually committed.

In the United States, a person who learns of the crime and gives some form of assistance before the crime is committed is known as an "accessory before the fact." A person who learns of the crime after it is committed and helps the criminal to conceal it, or aids the criminal in escaping, or simply fails to report the crime, is known as an "accessory after the fact." A person who does both is sometimes referred to as an "accessory before and after the fact," but this usage is less common.

In some jurisdictions, criminal "facilitation" laws do not require that the primary crime be actually committed as a prerequisite for criminal liability. These include state statutes making it a crime to "provide" a person with "means or opportunity" to commit a crime, "believing it probable that he/she is rendering aid to a person who intends to commit a crime."

To be convicted of an accessory charge, the accused must generally be proved to have had actual knowledge that a crime was going to be, or had been, committed. Furthermore, there must be proof that the accessory knew that his or her action, or inaction, was helping the criminals commit the crime, or evade detection, or escape. A person who unknowingly houses a person who has just committed a crime, for instance, may not be convicted of being an accessory offense because they did not have knowledge of the crime.