The Palace of Westminster, the medieval royal palace used as the home of the British parliament, was largely destroyed by fire on 16 October 1834. The blaze was caused by the burning of small wooden tally sticks which had been used as part of the accounting procedures of the Exchequer until 1826. The sticks were disposed of carelessly in the two furnaces under the House of Lords, which caused a chimney fire in the two flues that ran under the floor of the Lords' chamber and up through the walls.

The resulting fire spread rapidly throughout the complex and developed into the largest conflagration in London between the Great Fire of 1666 and the Blitz of the Second World War; the event attracted large crowds which included several artists who provided pictorial records of the event. The fire lasted for most of the night and destroyed a large part of the palace, including the converted St Stephen's Chapelthe meeting place of the House of Commonsthe Lords Chamber, the Painted Chamber and the official residences of the Speaker and the Clerk of the House of Commons.

The actions of Superintendent James Braidwood of the London Fire Engine Establishment ensured that Westminster Hall and a few other parts of the old Houses of Parliament survived the blaze. In 1836 a competition for designs for a new palace was won by Charles Barry. Barry's plans, developed in collaboration with Augustus Pugin, incorporated the surviving buildings into the new complex. The competition established Gothic Revival as the predominant national architectural style and the palace has since been categorised as a UNESCO World Heritage Site, of outstanding universal value.

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.