In the criminal law of many nations, necessity may be either a possible justification or an exculpation for breaking the law. Defendants seeking to rely on this defense argue that they should not be held liable for their actions as a crime because their conduct was necessary to prevent some greater harm and when that conduct is not excused under some other more specific provision of law such as self defense. Except for a few statutory exemptions and in some medical cases there is no corresponding defense in English law for murder. For example, a drunk driver might contend that he drove his car to get away from a kidnap (cf. North by Northwest). Most common law and civil law jurisdictions recognize this defense, but only under limited circumstances. Generally, the defendant must affirmatively show (i.e., introduce some evidence) that (a) the harm he sought to avoid outweighs the danger of the prohibited conduct he is charged with; (b) he had no reasonable alternative; (c) he ceased to engage in the prohibited conduct as soon as the danger passed; and (d) he did not himself create the danger he sought to avoid. Thus, with the "drunk driver" example cited above, the necessity defense will not be recognized if the defendant drove further than was reasonably necessary to get away from the kidnapper, or if some other reasonable alternative was available to him. However case law suggests necessity is narrowed to medical cases. Necessity as a defense to criminal acts conducted to meet political ends was rejected in the case of United States v. Schoon. In that case, 30 people, including appellants, gained admittance to the IRS office in Tucson, where they chanted "keep America's tax dollars out of El Salvador," splashed simulated blood on the counters, walls, and carpeting, and generally obstructed the office's operation. The court ruled that the elements of necessity did not exist in this case.

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