Ceefax (/ˈsiːfæks/) was the world's first teletext information service and a forerunner to the current BBC Red Button service. Ceefax was started by the BBC in 1974 and ended, after 38 years of broadcasting, at 23:32:19 BST (11:32 PM BST) on 23 October 2012, in line with the digital switchover completion in Northern Ireland.

To receive a desired page of text on a teletext-capable receiver, the user entered a three-digit page number on the device. The selected page was displayed on the user's screen as it was transmitted, requiring a wait of several seconds. There were many pages to choose from, and they could be displayed either on a black background or superimposed over the broadcast programme picture. This latter feature made it technically possible for the first time for British broadcasters to transmit subtitles that could be turned on or off by the viewer, rather than as part of the broadcast image.

During the late 1960s, engineers Geoff Larkby and Barry Pyatt at the Designs Department (Television Group) of the BBC worked on an experimental analogue text transmission system. Its object was to transmit a printable page of text during the nocturnal "close-down" period of normal television transmission. Sir Hugh Carleton Greene, then Director General of the BBC, was interested in making farming and stock-market prices available as hard copy via the dormant TV transmitters. The remit received by BBC Designs Department was "the equivalent of one page of The Times newspaper to be transmitted during shut-down".

Their system employed a modified rotating-drum facsimile transmitter designed by Alexander Muirhead, and Larkby & Pyatt's own, unique, design of hard-copy printer. This printer used pressure-sensitive "till-roll" paper passing over a drum with a raised helix of steel wire.

The drum was synchronised with the transmission drum by means of the "Start of Page", and "Start of Line" information inherent in the Muirhead system. Printing was effected by a hardened steel blade driven by, initially, a loudspeaker-type moving coil, then by a printed-circuit coil, and finally by a special ceramic piezo element manufactured by Brush-Clevite. The combination of rotating helix and oscillating moving blade, with the till-roll paper moving linearly between them, enabled a raster to be drawn on the paper.

The idea was later taken up again, this time in digital and on-screen form, under the new name of CEEFAX, meaning "see facts". The new system was announced in October 1972, and following test transmissions in 1972–74, the Ceefax system went live on 23 September 1974 with thirty pages of information. Created in the United Kingdom in the early 1970s by the Philips Lead Designer for VDUs John Adams, his design was given to the BBC so they could start transmission. The BBC were working on ways of providing televisual subtitles for deaf people, it was the first teletext system in the world. James Redmond, the BBC's Director of Engineering at the time, was a particular enthusiast. Other broadcasters soon took up the idea, including the Independent Broadcasting Authority (IBA), who had developed the incompatible ORACLE teletext system, at around the same time. Before the Internet and the World Wide Web became popular, Ceefax pages were often the first location to report a breaking story or headline.

After technical negotiations, the two broadcasters settled in 1974 on a single standard, different from both Ceefax and ORACLE, which ultimately developed into World System Teletext (1976), and which remained in use for analogue broadcasts until 2012. The display format of 24 rows by 40 columns of characters was also adopted for the Prestel system.

The technology became the standard European teletext system and replaced other standards, including the Antiope system formerly used in France.