Keep Calm and Carry On was a motivational poster produced by the Government of the United Kingdom in 1939 in preparation for World War II. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities. Although 2.45 million copies were printed, and the Blitz did in fact take place, the poster was only rarely publicly displayed and was little known until a copy was rediscovered in 2000 at Barter Books, a bookshop in Alnwick. It has since been re-issued by a number of private companies, and has been used as the decorative theme for a range of products.

Evocative of the Victorian belief in British stoicism – the "stiff upper lip", self-discipline, fortitude, and remaining calm in adversity – the poster has become recognised around the world. It was thought that only two original copies survived until a collection of approximately 15 was brought in to the Antiques Roadshow in 2012 by the daughter of an ex-Royal Observer Corps member. A few further examples have come to light since.

During 1938, newspapers were sold with a "Keep Calm and Dig" poster. The Keep Calm and Carry On poster was designed by the Ministry of Information from 27 June to 6 July 1939. It was produced as one of three "Home Publicity" posters (the others read "Your Courage, Your Cheerfulness, Your Resolution Will Bring Us Victory" and "Freedom Is in Peril / Defend It With All Your Might"). Each poster showed the slogan under a representation of a Tudor Crown (a symbol of the state). Keep Calm was intended to be distributed to strengthen morale in the event of a wartime disaster, such as mass bombing of major cities using high explosives and poison gas, which was widely expected within hours of an outbreak of war.

A career civil servant named A. P. Waterfield came up with "Your Courage" as one of several suggestions to be used as "a rallying war-cry that will bring out the best in everyone of us and put us in an offensive mood at once". Others involved in the planning of the early posters included John Hilton, Professor of Industrial Relations at Cambridge University, responsible overall as Director of Home Publicity; William Surrey Dane, managing director at Odhams Press; Gervas Huxley, former head of publicity for the Empire Marketing Board; William Codling, controller of HMSO; Harold Nicolson, MP; W. G. V. Vaughan, who became Director of the General Production Division (GPD); H. V. Rhodes, who later wrote an occasional paper on setting up a new government department; Ivison Macadam; "Mr Cruthley"; and "Mr Francis". Ernest Wallcousins was the artist tasked with creating the poster designs.

Detailed planning for the posters had started in April 1939 and the eventual designs were prepared after meetings between officials from the Ministry of Information and HM Treasury on 26 June 1939, and between officials from the Ministry of Information and HMSO on 27 June 1939. Roughs of the poster were completed on 6 July 1939, and the final designs were agreed by the Home Secretary Samuel Hoare, 1st Viscount Templewood on 4 August 1939. Printing began on 23 August 1939, the day that Nazi Germany and the USSR signed the Molotov–Ribbentrop Pact, and the posters were ready to be placed up within 24 hours of the outbreak of war.

The posters were produced in 11 different sizes, ranging from 15 × 10 inches (38 × 25 cm) up to large 48-sheet versions. The background colour was either red or blue. The lettering was probably hand-drawn by Wallcousins: it is similar, but not identical, to humanist sans-serif typefaces such as Gill Sans and Johnston.

Almost 2,500,000 copies of Keep Calm and Carry On were printed between 23 August and 3 September 1939 but while Your Courage and Freedom is in peril were both widely distributed, Keep Calm was not sanctioned for immediate public display. It was instead decided that copies should remain in "cold storage" for use after serious air raids (with resources transferred to Your Courage and Freedom is in Peril). Copies of Keep Calm and Carry On were retained until April 1940, but stocks were then pulped as part of the wider Paper Salvage campaign. Some copies do appear to have been displayed, but such instances were rare and unauthorised. An October 1940 edition of the Yorkshire Post reports the poster hung in a shop in Leeds; a photograph discovered in 2016 shows it on the wall of a government laboratory in Bedfordshire; and a large version displayed in a pub appears in a 1941 photograph by Cecil Beaton. An example also features in a drawing of a London Underground station by Floyd MacMillan Davis, published in Life magazine in 1944, suggesting a more widespread distribution.

The posters had been conceived on the assumption that enemy attacks of the civilian population would begin as soon as war was declared, and that there would be a great need for "a copious issue of general reassurance material". In practice, the initial poster campaign co-incided with the Phoney War, and thus to a population as yet completely unaffected by direct encounter with the enemy.