The GIUK gap is an area in the northern Atlantic Ocean. Its name is an acronym for Greenland, Iceland, and the United Kingdom, the gap being the open ocean between these three landmasses. The term is typically used in military topics.

The GIUK gap is particularly important to the Royal Navy, as any attempt by northern European forces to break into the open Atlantic would have to do so either through the easily defended English Channel, the Straits of Gibraltar past British shore guns, or through one of the exits on either side of Iceland. Only France and Portugal had direct access to the ocean in a way that could not be easily blocked at a choke point by the Royal Navy.

During World War II the gap was used by German ships to break out from their bases in northern Germany and Norway in an attempt to attack convoys, but these actions were generally unsuccessful due to blocking efforts in the North Sea and the GIUK gap. The Germans were aided tremendously with the fall of France, when they were able to base their submarines on the French coast. Between 1940 and 1942 the area between Iceland and Greenland was one of the few areas that RAF patrol bombers couldn't reach, and thus became the center for considerable action.

The origin of the term "gap" can be traced to this period, when there was a gap in air coverage known as the "Greenland air gap". The gap was eventually closed in 1943 with longer-ranged versions of aircraft such as the Shorts Sunderland and B-24 Liberator, making submarine actions in the Atlantic nearly impossible.

The gap again became the center of naval planning in the 1950s, as it would be the only available outlet into the ocean for Soviet submarines operating from their bases on the Kola Peninsula. The US and British based much of their post-war naval strategy on blocking the gap, eventually installing a chain of underwater listening posts right across it, known as SOSUS. The primary concern that was if the Cold War "turned hot" naval convoys reinforcing Europe from the United States would suffer unacceptable losses if Soviet submarines were allowed to operate in the North Atlantic.

The Royal Navy's primary mission during the Cold War, excluding the nuclear deterrent role, was that of Anti-Sub-Warfare (ASW). The development of the Invincible class CVS's (anti-submarine carriers) were part of this doctrine with their primary mission being anti submarine warfare using the Sea King. The Type 23 was to be a pure ASW platform, its mission expanded following the Falklands War.