Sir Clifford Paterson was the director of the GEC Research Laboratories at Wembley from their foundation in 1919 until his death in 1948. This book contains extracts from the daily diary that he kept from the beginning of September 1939 until May 1945. Paterson records work in the laboratories and his own wider role in the planning and organization of the scientific war effort, against the background of progress with the War and the personal problems experienced by members of his staff. There are accounts of visits with ministers, senior service officers, and officials in government departments and research establishments. The diary outlines progress on a wide variety of projects including the development of the cavity magnetron, airborne radar, radio countermeasures, centimeter wave propagation, as well as the development of diamond dies and quality control. A recurring theme is the development and production in the laboratories of more than 300,000 thermionic valves of 45 new types.