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 most recently presented by Colin Murray, assisted by Rachel Riley with lexicographer Susie Dent. It was the first programme to be broadcast on Channel 4 and 92 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), ran on French television almost continuously from 1965 until 2024.

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.

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 on 26 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 his retirement in 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 after 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 and originally appeared as "vital statistician", a role in which she alternated with Dr Linda Barrett until Barrett's departure after Series 2. Letters were placed on the board initially by Cathy Hytner, who was then followed by Karen Loughlin in Series 14 (1987) and Lucy Summers for Series 17 in early 1989. Numbers were originally put up on the board by Beverley Isherwood and this was taken over by Hytner mid-way through Series 3. From the start of Series 18 in July 1989, after Summers left the show, Vorderman began putting up the letters and numbers on a permanent basis. After 26 years, she left the show in December 2008 (at the same time as O'Connor) and was replaced by Rachel Riley who has appeared in the role since January 2009. Dr Anne-Marie Imafidon stood in for Riley from December 2021 to March 2022 whilst Riley was on maternity leave. Dr Tom Crawford, an Oxford and Cambridge university lecturer, stood in for Riley for three weeks in early 2025, making his first appearance on 24 February as the show's first-ever male arithmetician.

Susie Dent first appeared on the show in June 1992 as one of the regularly rotated lexicographers; however, it was not until 2003 that she became a recurring member of the now reduced on-screen team. Since 2005 she is the sole lexicographer on the show and has been credited as a co-presenter since January 2015; in each episode she appears in "Dictionary Corner" alongside a celebrity guest who changes from week to week. The other more notable past lexicographers, that have appeared in more than 100 episodes, include Catherine Clarke, Damian Eadie, Alison Heard, Mark Nyman, Richard Samson, Julia Swannell, Della Thompson, Freda Thornton and Yvonne Warburton.

The timeline below includes all main, co and guest presenters alongside notable past lexicographers as stated above.