Big Ben is the name of the biggest bell in the Clock Tower of the Houses of Parliament, also known as the Palace of Westminster. Although the term 'Big Ben' technically applies to the bell only, it is commonly used to refer to the whole Clock Tower, which is the world's largest four-faced chiming clock and the world's third largest free-standing clock tower.

Big Ben is the third heaviest bell in the UK and is used to chime the hour. It celebrates its 150th birthday in 2009.

The clock tower was added by Sir Charles Barry, the architect entrusted with creating a Gothic-style Palace after fire destroyed the original palace that occupied the site. In fact, for the design of the Clock Tower itself, he turned to his colleague Augustus Pugin; this was to be Pugin's last design before his descent into madness and death. At the time of his completion of his designs Pugin wrote: "I never worked so hard in my life [as] for Mr Barry... tomorrow I render all the designs for finishing his bell tower, and it is beautiful."

The tower is over 96 metres tall. Due to ground changes since its construction, most significantly due to tunneling for the Jubilee Line extension, the Tower leans slightly to the north-west - by roughly 20 centimetres.