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The Western Approaches is an approximately rectangular area of the Atlantic ocean lying immediately to the west of the British Isles. The north and south boundaries are defined by the corresponding extremities of the British Isles. The coast of the mainland forms the eastern side and the western boundary is the 30 degree meridian, which passes through Iceland. The area is particularly important to the United Kingdom, because many of its larger shipping ports lie within it.
The term is most commonly used when discussing naval warfare, notably during the First World War and Battle of the Atlantic during the Second World War in which Nazi Germany's Kriegsmarine attempted to blockade the United Kingdom using submarines (U-boats) operating in this area. Since almost all shipping to and from the United Kingdom passed through this area, it was an excellent hunting ground and had to be heavily defended.