It was a great honour to be entrusted by the family of Alastair Denniston with the task of telling the story of his life and career in signals intelligence. I would like to thank them all, particularly his niece Libby, for their support and providing me with documents and photographs. AGD’s son Robin had researched his father’s career for a number of years. I am grateful to Robin’s son Nick for providing me with all of his father’s research material which he had faithfully kept. When I began this project, Tony Comer, the GCHQ Departmental Historian, offered his support. I am grateful to Tony for reviewing an early draft of the book and providing an official statement on behalf of GCHQ as well as several previously unpublished photographs. I would like to thank my daughters for their ever-present support and love. Finally, I would like to thank the woman in my life for making every day worth living.