Countdown is a British game show involving word and mathematical tasks that began airing in November 1982. It is broadcast on Channel 4 and is currently presented by Colin Murray, assisted by Rachel Riley with lexicographer Susie Dent. It was the first programme to be broadcast on Channel 4 and 89 series have been broadcast since its debut on 2 November 1982. With over 8,000 episodes, Countdown is one of Britain's longest-running game shows; the original French version, Des chiffres et des lettres (Numbers & Letters), has been running on French television continuously since 1965. Countdown was produced by Yorkshire Television and was recorded at The Leeds Studios for 27 years, before moving to the Manchester-based Granada Studios in 2009. Following the development of MediaCityUK, Countdown moved again in 2013 to the new purpose-built studios at Dock10 in Greater Manchester.

The programme was presented by Richard Whiteley for 23 years until his death in June 2005. It was then presented by Des Lynam from October 2005 until December 2006, Des O'Connor from January 2007 until December 2008, Jeff Stelling from January 2009 until December 2011 and Nick Hewer from January 2012 until June 2021, with Colin Murray standing in for Nick Hewer during part of the COVID-19 pandemic. The programme was then presented by Anne Robinson from June 2021 until July 2022. Murray then returned on 14 July as a stand-in host. On 25 July 2022, it was announced that Les Dennis would guest host the show from 4 to 15 August followed by Jenny Eclair from 16 to 19 August because Murray tested positive for COVID-19. Guest hosts returned later that year as part of the show's 40th anniversary celebrations, with Floella Benjamin, Richard Coles, Trevor McDonald (for a second time) and Moira Stuart each hosting one week's episodes. Murray was announced as the programme's new permanent host in January 2023.

In the early years, the show had multiple assistant presenters, including Carol Vorderman who was hired for Series 1 to check calculations in the numbers round and her role expanded to include placing the number and letter tiles. She left in 2008 at the same time as O'Connor and was replaced by Rachel Riley. Dr Anne-Marie Imafidon stood in for Riley from December 2021 to March 2022 whilst she was on maternity leave. Susie Dent has been the permanent lexicographer since 2003 and appears in "Dictionary Corner" alongside a celebrity guest.

The two contestants in each episode compete in three game types: ten letters rounds, in which they attempt to make the longest word possible from nine randomly chosen letters, four numbers rounds, in which they must use arithmetic to reach a random target figure from six other numbers and the conundrum, a buzzer round in which the contestants compete to solve a nine-letter anagram. During the series heats, the winning contestant returns the next day until they either lose or retire with eight wins as an undefeated "Octochamp". The best eight contestants are invited back for the series finals, which are decided in knockout format. Contestants of exceptional skill have received national media coverage and the programme, as a whole, is widely recognised and parodied within British culture.