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 federal government's 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, designed to prevent a person from detonating it or removing its safety features. More recent innovations have included encrypting the firing parameters it is programmed with, which must be decrypted to properly detonate the warhead, and anti-tamper systems which intentionally mis-detonate the weapon if its other security features are defeated, destroying it without giving rise to a nuclear explosion.

Permissive action links were developed in the United States in a gradual process from the first use of atomic weapons to the early 1960s. In 1953 the United States Atomic Energy Commission and the Department of Defense signed the Missiles and Rockets Agreement, which paved the way for the development and implementation of PALs. Certain national laboratories, under the auspices of the AEC, would develop and produce nuclear weapons, while the responsibility for the use and deployment remained with the military. The laboratories were also free to conduct their own research in the field of arms control and security. The thinking behind this was that if the government would ever be interested in such a security device, the research and development of prototypes would already be well advanced. At the beginning of the 1960s, the desire for the usage of such a system grew for both political and technological reasons.

Newer nuclear weapons were less complex in operation, relatively mass-produced (and therefore predictably similar), and less cumbersome to arm and use than previous designs. Accordingly, new methods were necessary to prevent their unauthorized use. As the Cold War came to a head in the 1960s, the government felt it best not to leave the use of nuclear weapons in the hands of possibly-renegade generals, including the commander of Strategic Air Command (SAC). Without Permissive Action Links, each nuclear weapon was effectively under the independent control of one person, the general under whose command it happened to fall.

I used to worry about the fact that [General Power] had control over so many weapons and weapon systems and could, under certain conditions, launch the force. Back in the days before we had real positive control [i.e., PAL locks], SAC had the power to do a lot of things, and it was in his hands, and he knew it.

— General Horace M. Wade, (at that time subordinate of General Power),

In order to protect its NATO allies, the United States had stationed various nuclear weapons overseas; these weapons were thus at least under the partial control of the hosting allied state. This was especially concerning to the United States Congress, as control of these weapons by a third party was in violation of U.S. federal law.^{[citation needed]} Added to this was that some of the allies were considered potentially unstable—particularly West Germany and Turkey. There was considerable concern that in one of these countries the instructions of the civilian leadership of the host country could overrule that country's military. In addition, the U.S. realized that in the event of war, parts of West Germany would be overwhelmed early on, and nuclear weapons stationed there could fall into the hands of the Soviet Union.