Hut 6 was a wartime section of the Government Code and Cypher School (GC&CS) at Bletchley Park, Buckinghamshire, Britain, tasked with the solution of German Army and Air Force Enigma machine cyphers. Hut 8, by contrast, attacked Naval Enigma. Hut 6 was established at the initiative of Gordon Welchman, and was run initially by Welchman and fellow Cambridge mathematician John Jeffreys.
Hut 6 was partnered with Hut 3, which handled the translation and intelligence analysis of the raw decrypts provided by Hut 6.
Hut 6 was originally named after the building in which the section was located. Welchman says the hut was 20 yards (18m) long by 10 yards (9m) wide, with two large rooms at the far end – and no toilets. Staff had to go to another building. Irene Young recalled that she "worked in Room 82, though in typical Bletchley fashion there were not eighty-one rooms preceding it". She was glad to move from the Decoding Room "where all the operators were constantly having nervous breakdowns on account of the pace of work and the appalling noise" to the Registration Room which arranged intercepts according to callsign and frequency 
As the number of personnel increased, the section moved to additional buildings around Bletchley Park, but its name was retained, with each new location also being known as 'Hut 6'. The original building was then renamed 'Hut 16'.
John Jeffreys was initially in charge of the Hut with Gordon Welchman until May 1940; Jeffreys was diagnosed ill in 1940, and died in 1944. Welchman became official head of section until autumn 1943, subsequently rising to Assistant Director of Mechanisation at Bletchley Park. Hugh Alexander, was a member February 1940 – March 1941 before moving to become head of Hut 8. Stuart Milner-Barry joined early 1940 and was in charge from autumn 1943 to the end of the war.
One codebreaker concerned with Cryptanalysis of the Enigma, John Herivel, discovered what was soon dubbed the Herivel tip or Herivelismus. For a brief but critical few months from May 1940, the "tip", in conjunction with operating shortcomings or "cillies", were the main techniques used to solve Enigma. The "tip" was an insight into the habits of the German machine operators allowing Hut 6 to easily deduce part of the daily key.
In 1942, Welchman recruited fellow Marlborough Collegers, Bob Roseveare and Nigel Forward. Roseveare started in the Watch working on Luftwaffe messages before moving to the Quatch, a small backroom group that decoded non-current messages.
In Hut 6 were the Machine Room, plus the Decoding Room and Registration Room with mainly female staff under Harold Fletcher, a school and university friend of Gordon Welchman. In 2014 one of these female staff, Mair Russell-Jones, published a posthumous memoir of her work there.
Other notable individuals include:
- Alexander Aitken, New Zealand and Scottish mathematician
- James Macrae Aitken, Scottish chess player
- Madge E. Allen
- Dennis Babbage
- Stephen Banister, English cricketer and civil servant
- Clarence Barasch, American observer to British Intelligence
- Asa Briggs, English historian
- June Canney, Registration Room later Welchman's secretary 
- John Coleman and George Crawford, Intercept Control Room.
- Harold Fletcher, administrator
- Edna Garbutt, member of A watch
- Jane Hughes, later Jane Hughes Fawcett, who decoded the message that led to the sinking of the German battleship Bismarck
- Ione Jay, (Ione Roseveare) wrote an account of her experiences there
- Oliver Lawn, married 1948 Sheila who worked elsewhere at BP, Scottish country dancer.
- Reginald H. Parker, discoverer of "Parkerismus"
- David Rees, University of Exeter mathematics professor
- Derek Taunt, Cambridge research student
- Irene Young, University of Edinburgh, Decoding Room then Registration Room
- Helen C. Stanley-Baker, Oxford mathematics student
- Ann Katharine Mitchell (nee Williamson), Oxford mathematics graduate
- Sheila Dunlop (later Lady Killanin) 
- Ralph Erskine, Barry, Sir (Philip) Stuart Milner- (1906–1995), Oxford Dictionary of National Biography, 2004.
- Young (1990) p 74
- Briggs (2011), p 84-85
- Welchman (1982) p 103-4
- Robin Turner, "Bletchley Park uncovered: The secret life of the Welsh World War II codebreaker", Wales Online, July 27, 2014.
- J. M. Whittaker, ‘Aitken, Alexander Craig (1895–1967)’, rev. Anita McConnell, Oxford Dictionary of National Biography, Oxford University Press, 2004; online edn, May 2006.
- "Bletchley Park | Roll of Honour — undefined". Bletchley Park. Bletchley Park Trust. Retrieved 10 October 2017.
- Welchman (1982) p 86-7
- Welchman (1982) p 84
- Briggs (2011) p 8, 45
- Bletchley Park Summary of Service, 2013
- Head of Registration Room
- Asa Briggs, Secret Days: Code-breaking in Bletchley Park (2011, Frontline Books, London) pp. 84–85 ISBN 978-1-84832-615-6
- Stuart Milner-Barry, "Hut 6: Early days", pp. 89–99 in Codebreakers: The Inside Story of Bletchley Park, edited by F. H. Hinsley, and Alan Stripp, Oxford University Press, 2003
- Mair and Gethin Russell-Jones, My Secret Life in Hut Six: One woman’s experiences at Bletchley Park (2014; Lion Hudson, Oxford) ISBN 978 0 7459 5664 0
- Derek Taunt, "Hut 6: 1941-1945", pp. 100–112 in Codebreakers: The Inside Story of Bletchley Park', edited by F. H. Hinsley, and Alan Stripp, Oxford University Press, 2003
- Gordon Welchman, The Hut Six Story: Breaking the Enigma Codes (1982: London, Allen Lane & New York, McGraw-Hill) ISBN 0-7139-1294-4
- Gordon Welchman, The Hut Six Story: Breaking the Enigma Codes (1997: Cleobury Mortimer, Baldwin) ISBN 978-0-947712-34-1
- Irene Young, Enigma Variations: A Memoir of Love and War (1990, Mainstream Publishing, Edinburgh) ISBN 1-85158-294-0
- Alan Turing, Gordon Welchman, Hugh Alexander, Stuart Milner-Barry. Letter to Winston Churchill (1941)