Alan Turing (1912-1954) made two outstandingly original contributions to the development of computer science: his paper On Computable Numbers (1936) outlined a theoretical universal machine (or Turing machine), an idea which was more fully developed in his brilliant design for the Automatic Computing Engine (ACE), built after the Second World War at the National Physical Laboratory. He was also an important figure in the Colossus codebreaking operations at Bletchley Park during the War; made contributions to programming the Manchester University Mark I computer in the early 1950s; researched the subject of morphogenesis in plants at Manchester University; and from time to time explored the problem of machine intelligence. His impact on computer science, however, remains controversial. Turing's unorthodox personality and the brilliance of his ideas have many admirers; yet some computer scientists are sceptical about the actual impact of his ideas which, partly due to Turing's personality and social factors, were not widely disseminated.