The Vanguard class is a class of nuclear-powered ballistic missile submarines (SSBNs) in service with the Royal Navy. The class was introduced in 1994 as part of the Trident nuclear programme, and comprises four vessels: Vanguard, Victorious, Vigilant and Vengeance, built between 1986 and 1999 at Barrow-in-Furness by Vickers Shipbuilding and Engineering, now owned by BAE Systems. All four boats are based at HM Naval Base Clyde , 40 km (25 mi) west of Glasgow, Scotland.

A nuclear submarine is a submarine powered by a nuclear reactor, but not necessarily nuclear-armed. Nuclear submarines have considerable performance advantages over "conventional" submarines. Nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for conventional submarines. The large amount of power generated by a nuclear reactor allows nuclear submarines to operate at high speed for long periods, and the long interval between refuelings grants a range virtually unlimited, making the only limits on voyage times being imposed by such factors as the need to restock food or other consumables.