John Douglas Cockcroft was born in Langfield, Yorkshire, 27 May 1897, the son of John Arthur Cockcroft and Annie Maude Fielden. He was educated at Todmorden Secondary School, 1909-14; Manchester University, 1914-15; Manchester College of Technology, 1919-20; and St John's College, Cambridge, 1922-4. He married Eunice Elizabeth Crabtree, 1925, with whom he had two sons and four daughters. He was a signaller in the Royal Field Artillery, 1915-18, and an engineering apprentice at the Metropolitan-Vickers Company, 1920-22. He worked as a research student in the Cavendish Laboratory, Cambridge University, 1924-8, and subsequently held posts there as a Demonstrator and a Lecturer, 1928-39, taking charge of the Royal Society Mond Laboratory, Cambridge, 1935. From 1928, he worked on the acceleration of protons by high voltages and, in 1932, he and Ernest Walton succeeded in splitting atomic nuclei. He was a fellow of St John's College, Cambridge, 1928-46, and Jacksonian Professor of Natural Philosophy, Cambridge University, 1939-46. He was Assistant Director of Scientific Research in the Ministry of Supply, 1939-40, working on the application of radar to coastal and air defences; Chief Superintendent of the Air Defence Research and Development Establishment, 1940-4; Director of the Montreal and Chalk River Laboratories, Canada, 1944-6; and Director of the Atomic Energy Research Establishment, Harwell, 1946-58. He was Chairman of the Defence Research Policy Committee and a scientific adviser to the Ministry of Defence, 1952-4; a full-time member of the Atomic Energy Authority, 1954-9; and a member of the Advisory Council on Scientific Policy, until 1963. He was the first Master of Churchill College, Cambridge, 1959-67. He was an honorary fellow of St John's College, Cambridge, 1946-67; President of Manchester College of Science and Technology, 1961-7; and Chancellor of the Australian National University, 1961-5. He was awarded the Nobel Prize for Physics, 1951. He was awarded the CBE, 1944, a knighthood, 1948, the OM, 1957, and created KCB, 1953. He died at Churchill College on 18 September 1967. His publications include: editor, "The Organization of Research Establishments" (1965).