Big Ben is the nickname for the Great Bell of the Great Clock of Westminster, and, by extension, for the clock tower itself, which stands at the north end of the Palace of Westminster in London, England. Originally known simply as the Clock Tower, it was renamed Elizabeth Tower in 2012 to mark the Diamond Jubilee of Elizabeth II. The clock is a striking clock with five bells.

The tower was designed by Sir Charles Barry and Augustus Pugin in a Perpendicular Gothic Revival style and was completed in 1859. It is elaborately decorated with stone carvings and features symbols related to the four nations of the United Kingdom and the Anglo-Welsh Tudor dynasty. A Latin inscription celebrates Queen Victoria, in whose reign the palace was built. 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. The dials of the clock are 22.5 feet (6.9 m) in diameter.

The clock uses its original mechanism and was the largest and most accurate four-faced striking and chiming clock in the world upon its completion. It was designed by Edmund Beckett Denison and George Airy, the Astronomer Royal, and constructed by Edward John Dent and Frederick Dent. It is known for its reliability, and can be adjusted by adding or removing pre-decimal pennies from the pendulum. The Great Bell was cast by the Whitechapel Bell Foundry and weighs 13.5 long tons (13.7 tonnes; 15.1 short tons). Its nickname may be derived from Sir Benjamin Hall, who oversaw its installation, or heavyweight boxing champion Benjamin Caunt. There are four quarter bells, which chime on the quarter hours.

Big Ben is a British cultural icon. 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. The clock and tower were renovated between 2017 and 2021, during which the bells remained silent with few exceptions.

Elizabeth Tower, originally named the Clock Tower, and popularly known as "Big Ben", was built as a part of Charles Barry's design for a new Palace of Westminster after the old palace was largely destroyed by fire on 16 October 1834. Although Barry was the chief architect of the neo-gothic palace, he turned to Augustus Pugin for the design of the Clock Tower, which resembles earlier Pugin designs, including one for Scarisbrick Hall in Lancashire. Construction of the tower began on 28 September 1843. The building contractors were Thomas Grissell and Morton Peto. An inscribed trowel in the Parliamentary Archives records that Emily, sister of Peto's daughter-in-law, was given the honour of laying the first stone. It was Pugin's last design before his descent into mental illness and death in 1852, and Pugin himself wrote, at the time of Barry's last visit to him to collect the drawings: "I never worked so hard in my life for Mr Barry for tomorrow I render all my designs for finishing his bell tower and it is beautiful".