The Royal Navy of the United Kingdom is the oldest of HM Armed Forces (and is therefore known as the Senior Service). From the beginning of the 19th century until well into the 20th century it was the most powerful navy in the world, playing a key part in establishing the British Empire as the dominant world power from 1815 until the early 1940s. In World War II the Royal Navy operated almost 900 ships. During the Cold War it was transformed into a primarily anti-submarine force (Anti-submarine warfare), hunting for Soviet (Soviet Union) submarines, mostly active in the GIUK gap. With the collapse (dissolution of the Soviet Union) of the Soviet Union, its role for the 21st century has returned to focus on global expeditionary (Blue-water navy) operations.

The Royal Navy is a blue water navy and the second-largest navy of the NATO alliance, in terms of the combined displacement (Displacement (ship)) approx. convert450000LT (convert950000LT including the Royal Fleet Auxiliary) after the United States Navy. As of2010 there were 87 commissioned ships (List of active Royal Navy ships) in the Royal Navy, including aircraft carriers, a helicopter carrier (amphibious assault ship), landing platform docks, ballistic missile submarines, nuclear fleet submarines (Nuclear submarine), guided missile destroyers, frigates, mine counter-measures and patrol vessels. Twenty-two vessels of the Royal Fleet Auxiliary (RFA) also contribute to the Royal Navys order-of-battle. The Royal Navys ability to project power (power projection) globally is considered second only to the U.S. Navy. The Royal Navy maintains the United Kingdoms nuclear weapons (Nuclear weapons and the United Kingdom).

The Royal Navy is a constituent component of the Naval Service, which also comprises the Royal Marines, Royal Naval Reserve and Royal Marines Reserve. As of April 2009 the Royal Navy numbered approximately 39,100 Regular personnel of whom 7,500 were in the Royal Marines; in addition, there were 3,600 Volunteer Reserve personnel, giving a total of 42,700 personnel

The Royal Navy is also supported by the Royal Fleet Auxiliary, a civilian logistical support fleet which is owned and operated by the Ministry of Defence as part of the British Merchant Navy. The RFA primarily serves to replenish Royal Navy warships at sea, but also augments the Royal Navys amphibious warfare capabilities through its four Bay-class (Bay class landing ship dock) LSDs (Landing Ship Dock).