Blockbuster was the name given to several of the largest conventional bombs used in World War II by the Royal Air Force (RAF). The term Blockbuster was originally a name coined by the press and referred to a bomb which had enough explosive power to destroy a city block.

The bombs then called blockbusters were the RAF's 4000lb - also known as a cookie, 8000 and 12,000 lb (1,800, 3,600 and 5,400 kg) HC (High Capacity) bombs. These bombs had especially thin casings that allowed them to contain approximately three-quarters of their weight in explosive, the 4000 pounder containing over 3000 lb (1360 kg) of explosive filling. Most 'normal' bombs (termed Medium Capacity - MC - by the RAF) at that time contained only 50% explosive by weight, the rest being made up of the bomb casing. These bombs were designed for blowing the tiles off the roofs of buldings so that the smaller 4 lb (1.8 kg) incendiary bombs could reach the building interiors. These 'High Capacity' bombs were only used by the RAF, being too big to fit in the bomb-bays of other country's aircraft.

The first type of aircraft to carry cookies operationally was the Wellington but they later became part of the standard bombload of the RAF's heavy night bombers, as well as that of the Mosquitoes of the Light Night Striking Force, whose aircraft would sometimes visit Berlin twice in one night carrying cookies, flown by two different crews. The 8000 and 12,000 lb (3,600 and 5,400 kg) bombs, because of their large size, were only able to be carried by the Lancaster.

The 4000 lb (1,800 kg) 'cookie' was regarded as particularly dangerous, as, due to the airflow over the detonating pistols fitted in the nose, it would often explode even if dropped 'safe', which may account for the loss of Glenn Miller.

A blockbuster may also refer to a bomb designed to destroy a blockhouse. Bombs which can penetrate reinforced concrete of a blockhouse are also referred to as bunker busters.

The two World War II bombs which best fit the description of bunker busters are the Tallboy bomb 12,000 lb (5,400 kg) MC and the Grand Slam bomb 22,000 (10,000 kg) MC both designed by Barnes Wallis for the RAF. The Tallboy and Grand Slam bombs had a thick skin to withstand the initial impact of hitting hardened targets and were therefore classified as Medium Capacity (MC) bombs.

Blockbusters got larger as the war progressed from 4,000 to 12,000 lb (1,800 to 5,400 kg).