Big Brother is the British version of the international reality television franchise Big Brother created by producer John de Mol Jr. in 1997. Broadcast yearly from 18 July 2000 to 5 November 2018, and returned on 8 October 2023, the show follows the format of other national editions, in which a group of contestants, known as "housemates", live together in a specially constructed house that is isolated from the outside world. Live television cameras and personal audio microphones continuously monitor them. Throughout the competition, housemates are "evicted" from the house by public televoting. The last remaining housemate wins the competition and a cash prize. The series takes its name from the oppressive character known by that name in George Orwell's 1949 novel Nineteen Eighty-Four.

The series premiered in 2000 on Channel 4 and immediately became a ratings hit. It featured a 24-hour live feed in which fans could view inside the house at any time. Big Brother aired for eleven series on Channel 4, followed by one final special edition, Ultimate Big Brother, which ended in 2010. Channel 5 acquired the rights to the series, and relaunched it in 2011. On the day of the nineteenth series launch in 2018, Channel 5 announced that Big Brother would not be returning to the channel after the end of that series. In 2020, Channel 4 announced that previous episodes of Big Brother would be shown on E4 in a series titled Big Brother: Best Shows Ever to mark the 20th anniversary of the programme. Two years later, in August 2022, it was announced by ITV that Big Brother would return for a new series on ITV2 the following year. The show was relaunched for a second time in 2023. In November 2024, ITV announced the series would return for a third series in 2025.

The programme was initially presented by Davina McCall MBE from its inception in July 2000 until its cancellation by Channel 4 ten years later. McCall declined to return as presenter following the programme's move to Channel 5, and the role was taken up by former series winner Brian Dowling. He remained as presenter throughout the twelfth and thirteenth series. Emma Willis later replaced Dowling from the fourteenth series onwards until its cancellation from Channel 5. On 25 April 2023, ITV announced AJ Odudu and Will Best as hosts of both the main show and a "nightly spin-off" show. Marcus Bentley served as the off-screen narrator of all series since the show premiered in 2000. Big Brother has had numerous spin-off series occur since its premiere, most notably Celebrity Big Brother, a shorter version of the main series wherein the cast is composed solely of celebrities. Numerous other spin-off series that are not competition based have aired, with Dermot O'Leary, Russell Brand, George Lamb, Emma Willis and Rylan Clark all presenting spin-offs.