The Vanguard Class Submarines were designed to deploy the Trident Missile system to replace the aging Resolution Class which deployed the Polaris Missile system, Britain's initial nuclear deterrent. The boats were based largely on the American Ohio Class the major difference being that the Vanguard Class was designed with 16 missile tubes instead of the 24 missile tubes of the Ohio Class boats. Twice as heavy and twenty metres longer the new Vanguard Class Submarines had many advantages over the Resolution Class including more space, with a fourth deck, and less crew due to increased automation. They also have improved sonar and carry the Spearfish torpedo. The development of the Trident Missile system received much interest from Russian intelligence and in what was dubbed "The Hunt for Red December" Russian submarines trying to monitor Vanguard's sea trials in December 1993 were chased across the Norwegian Sea by British frigates and submarines. Upon embarking on her first operational patrol Vanguard was the subject of further Russian interest and it is rumoured that at least two Trafalgar Class submarines, one of which was HMS Torbay, escorted her as she left the Clyde approaches and headed into the open ocean. In 1993 it was announced that Devonport had won the contract to refit and refuel the four Vanguard Class Submarines, worth £5 billion. Since the announcement a large amount of money has been spent constructing the facilities needed for the refit including the conversion of a 90 year old battleship drydock into a modern earthquake proof refit complex. The four Vanguard Class Submarines are members of the 1st Submarine Squadron based at HM Naval Base Clyde.

The Vanguard Class can carry sixteen Trident II (D5) Missiles which have a range of 4,000 nautical miles (around 11,000km). Each missile is 13 metres long, weighs 60 tonnes and can carry up to twelve warheads each of which can be programmed to attack different targets. At any single time there is at least one Vanguard Class Submarine on deterrent patrol, its location a closely guarded secret known only by the senior officers onboard. Once they leave Faslane they submerge and do not resurface until they return.