The Twentieth Century Society (abbreviated as C20) is a Britishcharity which campaigns for the preservation of architectural heritage from 1914 onwards. The society's interests embrace buildings and artefacts that characterise 20th-century Britain. It is formally recognised as one of the National Amenity Societies, and as such is a statutory consultee on alterations to listed buildings within its period of interest, and must be notified of any proposed work to a listed building which involves any element of demolition.
The society was formed as the Thirties Society in 1979, the year in which the prominent "Thirties – British art and design before the War" exhibition was shown at the Hayward Gallery. The society was inspired by and loosely modelled on the Victorian Society, which aims to protect pre-1914 Victorian and Edwardian buildings. Though several modern movement buildings had been recommended for listing by Nikolaus Pevsner in 1970, it was felt by Marcus Binney, John Harris and Sir Simon Jenkins that much more needed to be done.Bevis Hillier was the first president, and Clive Aslet the first honorary secretary. In 1992, the society changed its name to the Twentieth Century Society, as it was felt that "Thirties Society" failed to indicate its interest in the protection of buildings from other periods as well.
In 1985–6 the society campaigned against the wholesale replacement of the traditional K-series red telephone boxes designed by Sir Giles Gilbert Scott between 1926 and 1936. The campaign attracted much public support, and led to a number of boxes being listed.
In the 1980s and 1990s the society campaigned repeatedly for the preservation and listing of Bankside Power Station, designed by Sir Giles Gilbert Scott, constructed in two phases in 1947–52 and 1958–63, and decommissioned in 1981. The campaign was ultimately successful, and the building reopened as the Tate Modern art gallery in 2000.