The Great Clock of Westminster, usually referred to as Big Ben, which overlooks this Conference Centre was installed in the St. Stephens Tower at the Palace of Westminster in 1859. When the clock stopped abruptly in 1976 there was almost universal interest and concern. A fatigue failure in a vital component led to the destruction of one third of the mechanism of the clock. The concerted and minute examination of the structure, using a range of nondestructive techniques, that was carried out in the Autumn of 1976, ensured that the remaining two thirds of the clock was basically sound and thus contributed to other factors which led to the decision to rebuild the clock, rather than replace the original mechanism. The many replacement components that were required for the refurbishment were examined in 1976 and early in 1977. Since then a planned maintenance and inspection schedule has been agreed which ensures that those components which could lead to a failure of the clock are regularly examined or systematically replaced. The erection of scaffolding and a lift in order to renovate and clean the exterior of St. Stephens Tower in 1983-84 also offered the opportunity to examine the hands on the four faces of the clock and associated mechanisms. This paper describes the original design and the modifications imposed on the clockmaker during the building of the tower. -- AATA