By some inversion of Gresham's Law, it appears that first-class books on the British breaking of high-level German ciphers are now replacing the partial and popularized treatments of 2-3 years ago. Brigadier Lewin's comprehensive history of the whole Bletchley Park operation and of the impact of its ULTRA product on key decisions in World War II belongs on the shelf alongside Patrick Beesly on the naval side and R. V. Jones on the scientific (reviewed in Foreign Affairs respectively in July 1977 and October 1978). The author has drawn on the mass of messages made available in the past year in their deciphered form, on extensive and thorough interviews with both technical people and military and civilian users, and above all on his own deep background as an accomplished military historian of World War II. The result is incisive, clearly written, balanced, and critical - but at the same time understanding of the pressures under which intelligence had to be assessed and decisions taken. An enduring source for what was probably the most important sustained intelligence success in the history of human conflict.