A permissive action link (PAL) is an access control security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of a nuclear weapon. The United States Department of Defense definition is:

A device included in or attached to a nuclear weapon system to preclude arming and/or launching until the insertion of a prescribed discrete code or combination. It may include equipment and cabling external to the weapon or weapon system to activate components within the weapon or weapon system.

The earliest PALs were little more than locks introduced into the control and firing systems of a nuclear weapon, that would inhibit either the detonation, or the removal of safety features of the weapon. More recent innovations have included encrypted firing parameters, which must be decrypted to properly detonate the warhead, plus anti-tamper systems which intentionally mis-detonate the weapon, destroying it without giving rise to a nuclear explosion.