The Clock Tower is a turret clock structure at the north-eastern end of the Houses of Parliament building in Westminster, London. It is colloquially and popularly known as Big Ben, however this name actually belongs to the clock's main bell. The tower has also been incorrectly referred to as St. Stephen's Tower.

The tower was raised as a part of Charles Barry's design of a new palace, after the old Palace of Westminster was destroyed by fire on the night of October 16, 1834. The tower is designed in the Victorian Gothic style, and is 96.3 metres (316 feet) high.

The first 61 metres (200 feet) of the structure is the clock tower, consisting of brickwork with stone cladding; the remainder of the tower's height is a framed spire of cast iron. The tower is founded on a 15 by 15 metres (49 by 49 feet) raft, made of 3 metres (9 feet) thick concrete, at a depth of 7 metres (23 feet) below ground level. The tower has an estimated weight of 8,667 tonnes (9,553 tons). The four clock faces are 55 metres (180 feet) above ground.

Due to ground conditions present since construction, the tower leans slightly to the north-west, by roughly 220 millimetres (8.66 inches). Due to thermal effects it oscillates annually by a few millimetres east and west.

The Clock Faces were once large enough to allow the Clock Tower to be the largest four-faced clock in the world, but has since been outdone by the Allen-Bradley Clock Tower in Milwaukee, Wisconsin. The builders of the Allen-Bradley Clock Tower did not add chimes to the clock, so the Great Clock of Westminster still holds the title of the "World's largest four-faced chiming clock". The clock mechanism itself was completed by 1854, but the tower was not fully constructed until four years later in 1858.

The face of the Great Clock of Westminster. A 5 foot 4 inch person has been inserted into the picture at correct scale. The hour hand is 9 feet long and the minute hand is 14 feet longThe clock faces and dials were designed by Augustus Pugin. The clock faces are set in an iron framework 7 metres (21 feet) in diameter supporting 576 pieces of opal glass, rather like a stained glass window. Some of the glass pieces may be removed for inspection of the hands. The surround of the dials is heavily gilded. At the base of each clock face in gilt letters is the Latin inscription 'DOMINE SALVAM FAC REGINAM NOSTRAM VICTORIAM PRIMAM' meaning 'Lord save our Queen Victoria I'.

The clock became operational on September 7, 1859.

A 6 metre (20 foot) metal replica of the clock tower, known as Little Ben, complete with working clock, stands on a traffic island close to Victoria Station. Several turret clocks around the world are inspired by the look of the Great Clock, including the clock tower of the Gare de Lyon in Paris and the Peace Tower of the Parliament of Canada in Ottawa.