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.