Technical detail on how the German Enigma codes were broken, and lessons for today's military communications--drawn from the author's experience. Welchman, previously a Cambridge mathematician, was head of ""Hut Six"" at Britain's WW II cryptography-and-intelligence center, Bletchley Park, during the period (1940-41) when the codes were broken; as readers of Frederick Winterbotham, Ronald Lewin, Peter Calvocoressi, et al., will recognize, Hut Six was the BP cryptography section, which passed along decrypts to Hut Three, the intelligence section, for interpretation and transmission. The result was Ultra intelligence. One problem with Welchman's book is that Calvocoressi's Top Secret Ultra, also based on personal experience, presents a clear, succinct, cohesive account of the entire BP operation--whereas Welchman's text veers between family-album recollections of individuals (one new arrival ""was noted for the hole burnt in his trousers by a cigarette, or possibly by ash from a pipe"") and specialized information on the functioning of the Enigma machine (""With five wheels to choose from, the three wheels to be used in the scrambler could be selected in ten ways, and for each such selection. . .""). Welchman does explain the problems encountered at each stage (but so, more simply, does Calvocoressi); and he also has some points to make--about the German errors that enabled the problems to be surmounted, about how a nonprofessional like himself ""might find a way of defeating them,"" about the teamwork called for (not only within Hut Six, but also with the intercept stations and Hut Three). In the last third of the book, devoted to ""Today,"" he makes these and analogous points again--via historic examples and references to his later work as a (US) communications-systems executive. Much of this is in the form of injunctions to the military--but Welchman also has some pertinent things to say about the human failure at Three Mile Island (where, in contrast to Hut Six, there was no ""interdisciplinary cooperation""). Largely one man's legacy, and rambling--with some spot-material for cryptography adepts.