The doctrine of common purpose, common design, joint enterprise, joint criminal enterprise or parasitic accessory liability is a common law legal doctrine that imputes criminal liability to the participants in a criminal enterprise for all reasonable results from that enterprise. The common purpose doctrine was established in English law, and later adopted in other common-law jurisdictions including Scotland, Ireland, Australia, Trinidad and Tobago, the Solomon Islands, Texas, Massachusetts, the International Criminal Court, and the International Criminal Tribunal for the former Yugoslavia.

Common design also applies in the law of tort. It is a different legal test from that which applies in the criminal law. The difference between common designs in the criminal law and the civil law was illustrated in NCB v Gamble [1959] 1 QB 11 at 23, by Devlin LJ:

"the consequence [in the criminal law] is that selling a person a gun knowing that person will use it to kill someone else will make the seller an accessory to the murder but will not in itself make him liable in tort."

The difference applies in US law as well. The United States Supreme Court reached the same conclusion in Sony Corporation of America v Universal City Studios Inc. 464 US 417 (1984). The supply of equipment for copying video cassettes did not give rise to joint liability in tort for copyright infringement. There was no encouragement to copy music and therefore no liability as an accessory. The difference lies between mere knowledge at the point of sale and action combined with common intention: Metro-Goldwyn-Mayer Studios Inc. v Grokster Ltd. 545 US 913 (2005): see p. 931. A different result was reached in Metro-Goldwyn-Mayer by the US Supreme Court. There was a common design there because the defendants "distribute[d] a device with the object of promoting its use to infringe copyright". That test was satisfied because clear statements and positive steps were taken by the administrators of the website to encourage infringement.

A common application of the rule is to impute criminal liability for wounding a person to participants in a riot who knew, or were reckless as to knowing, that one of their number had a knife and might use it, despite the fact that the other participants did not have knives themselves. In England and Wales and certain other Commonwealth countries, this was the understanding of the courts until February 2016, when the Supreme Court of the United Kingdom and the Judicial Committee of the Privy Council jointly ruled in R v Jogee that it was wrong, and that nothing less than intent to assist the crime would do.