In criminal law, a conspiracy is an agreement between two or more persons to commit a crime at some time in the future. Criminal law in some countries or for some conspiracies may require that at least one overt act be undertaken in furtherance of that agreement, to constitute an offense. There is no limit on the number participating in the conspiracy and, in most countries, no requirement that any steps have been taken to put the plan into effect (compare attempts which require proximity to the full offense). For the purposes of concurrence, the actus reus is a continuing one and parties may join the plot later and incur joint liability and conspiracy can be charged where the co-conspirators have been acquitted or cannot be traced. Finally, repentance by one or more parties does not affect liability (unless, in some cases, it occurs before the parties have committed overt acts) but may reduce their sentence.

An unindicted co-conspirator, or unindicted conspirator, is a person or entity that is alleged in an indictment to have engaged in conspiracy, but who is not charged in the same indictment. Prosecutors choose to name persons as unindicted co-conspirators for a variety of reasons including grants of immunity, pragmatic considerations, and evidentiary concerns.

United States.

Conspiracy has been defined in the United States as an agreement of two or more people to commit a crime, or to accomplish a legal end through illegal actions. A conspiracy does not need to have been planned in secret to meet the definition of the crime.

Conspiracy law usually does not require proof of specific intent by the defendants to injure any specific person to establish an illegal agreement. Instead, usually the law requires only that the conspirators have agreed to engage in a certain illegal act.

In most U.S. jurisdictions, for a person to be convicted of conspiracy, not only must he or she agree to commit a crime, but at least one of the conspirators must commit an overt act (the actus reus) in furtherance of the crime. However, in United States v Shabani the U.S. Supreme Court ruled that this "overt act" element is not required under the federal drug conspiracy statute, 21 U.S.C. section 846.

The conspirators can be guilty even if they do not know the identity of the other members of the conspiracy.