Who Wants to Be a Millionaire? is a British television quiz show, created by David Briggs, Steven Knight and Mike Whitehill for the ITV network. The programme's format has contestants answering multiple-choice questions based on general knowledge, winning a cash prize for each question they answer correctly, with the amount offered increasing as they take on more difficult questions. If an incorrect answer is given, the contestant will leave with whatever cash prize is guaranteed by the last safety net they have passed, unless they opt to walk away before answering the next question with the money they had managed to reach. To assist in the quiz, contestants are given a series of "lifelines" to help answer questions.

The series originally aired from 4 September 1998 to 11 February 2014 and was presented by Chris Tarrant, airing a total of 592 episodes across 30 series. The original format was tweaked in later years, which included changing the number of questions asked, altering the payout structure, incorporating a time limit, and increasing the number of lifelines offered. After the original series ended, ITV decided to commemorate the 20th anniversary of the programme with a special series of episodes in 2018, produced by Stellify Media and hosted by Jeremy Clarkson. This proved a success with viewers and led to a revival of the programme, with new series being commissioned by the broadcaster and a spin-off ordered in 2022 called Fastest Finger First.

Over its history, the programme has seen a number of contestants manage to achieve the jackpot prize, but has also been involved in several controversies, including an attempt by a contestant to defraud the show of its top prize. Despite this, Who Wants to Be a Millionaire? became one of the most significant shows in British popular culture, ranking 23rd in a list of the 100 Greatest British Television Programmes compiled in 2000 by the British Film Institute. Its success led to the formation of an international franchise, with several countries featuring the same general format but with some variations in gameplay and lifelines provided.