Big Ben is the nickname for the Great Bell of the striking clock at the north end of the Palace of Westminster in London, England, and the name is frequently extended to refer also to the clock and the clock tower. The official name of the tower in which Big Ben is located was originally the Clock Tower, but it was renamed Elizabeth Tower in 2012 to mark the Diamond Jubilee of Elizabeth II.

The tower was designed by Augustus Pugin in a neo-Gothic style. When completed in 1859, its clock was the largest and most accurate four-faced striking and chiming clock in the world. The tower stands 316 feet (96 m) tall, and the climb from ground level to the belfry is 334 steps. Its base is square, measuring 40 feet (12 m) on each side. Dials of the clock are 22.5 feet (6.9 m) in diameter. All four nations of the UK are represented on the tower on shields featuring a rose for England, thistle for Scotland, shamrock for Ireland, and leek for Wales. On 31 May 2009, celebrations were held to mark the tower's 150th anniversary.Big Ben is the largest of the tower's five bells and weighs 13.5 long tons (13.7 tonnes; 15.1 short tons). It was the largest bell in the United Kingdom for 23 years. The origin of the bell's nickname is open to question; it may be named after Sir Benjamin Hall, who oversaw its installation, or heavyweight boxing champion Benjamin Caunt. Four quarter bells chime at 15, 30 and 45 minutes past the hour and just before Big Ben tolls on the hour. The clock uses its original Victorian mechanism, but an electric motor can be used as a backup.

The tower is a British cultural icon recognised all over the world. It is one of the most prominent symbols of the United Kingdom and parliamentary democracy, and it is often used in the establishing shot of films set in London. The clock tower has been part of a Grade I listed building since 1970 and a UNESCO World Heritage Site since 1987.

On 21 August 2017, a four-year schedule of renovation works began on the tower. Modifications have included adding a lift, re-glazing and repainting the clock dials, and upgrading lighting and repairing roof tiles among other improvements. With a few exceptions, such as New Year's Eve and Remembrance Sunday, the bells are to be silent until the work is completed in 2022. In December 2021, the last scaffolding was removed from the clock faces, revealing the restoration work that had taken place since 2017. In April 2022, the gantry supporting the scaffolding was removed, leaving the tower free of scaffolding.

The Palace of Westminster serves as the meeting place for both the House of Commons and the House of Lords, the two houses of the Parliament of the United Kingdom. Informally known as the Houses of Parliament after its occupants, the Palace lies on the north bank of the River Thames in the City of Westminster, in central London, England.

Its name, which derives from the neighbouring Westminster Abbey, may refer to several historic structures but most often: the Old Palace, a medieval building-complex largely destroyed by fire in 1834, or its replacement, the New Palace that stands today. The palace is owned by the monarch in right of the Crown and, for ceremonial purposes, retains its original status as a royal residence. Committees appointed by both houses manage the building and report to the Speaker of the House of Commons and to the Lord Speaker.

The first royal palace constructed on the site dated from the 11th century, and Westminster became the primary residence of the Kings of England until fire destroyed the royal apartments in 1512 (after which, the nearby Palace of Whitehall was established). The remainder of Westminster continued to serve as the home of the Parliament of England, which had met there since the 13th century, and also as the seat of the Royal Courts of Justice, based in and around Westminster Hall. In 1834 an even greater fire ravaged the heavily rebuilt Houses of Parliament, and the only significant medieval structures to survive were Westminster Hall, the Cloisters of St Stephen's, the Chapel of St Mary Undercroft, and the Jewel Tower.

In the subsequent competition for the reconstruction of the Palace, the architect Charles Barry won with a design for new buildings in the Gothic Revival style, specifically inspired by the English Perpendicular Gothic style of the 14th–16th centuries. The remains of the Old Palace (except the detached Jewel Tower) were incorporated into its much larger replacement, which contains over 1,100 rooms organised symmetrically around two series of courtyards and which has a floor area of 112,476 m2 (1,210,680 sq ft). Part of the New Palace's area of 3.24 hectares (8 acres) was reclaimed from the River Thames, which is the setting of its nearly 300-metre long (980 ft) façade, called the River Front. Augustus Pugin, a leading authority on Gothic architecture and style, assisted Barry and designed the interior of the Palace. Christian themes were integrated into the design. Construction started in 1840 and lasted for 30 years, suffering great delays and cost overruns, as well as the death of both leading architects; works for the interior decoration continued intermittently well into the 20th century. Major conservation work has taken place since then to reverse the effects of London's air pollution, and extensive repairs followed the Second World War, including the reconstruction of the Commons Chamber following its bombing in 1941.

The Palace is one of the centres of political life in the United Kingdom; "Westminster" has become a metonym for the UK Parliament and the British Government, and the Westminster system of government commemorates the name of the palace. The Elizabeth Tower, in particular, often referred to by the name of its main bell, Big Ben, has become an iconic landmark of London and of the United Kingdom in general, one of the most popular tourist attractions in the city, and an emblem of parliamentary democracy. Tsar Nicholas I of Russia called the new palace "a dream in stone". The Palace of Westminster has been a Grade I listed building since 1970 and part of a UNESCO World Heritage Site since 1987.