Given that until recently the British government refused to acknowledge the existence of its World War II–era code-breaking organization, this informative official history of the Government Communications Headquarters, or GCHQ, one of the United Kingdom’s leading intelligence agencies, is remarkable. Ferris’s narrative takes on the breaking of the Nazis’ Enigma code at Bletchley Park during World War II and the efforts to replicate that achievement during the Cold War. GCHQ now plays a major role in all areas of cybersecurity. Its activities, along with those of the U.S. National Security Agency, were compromised when a former NSA contractor, Edward Snowden, revealed them in 2013. Ferris’s account avoids sensationalism. It provides a careful judgment of Bletchley Park’s impact, points to how signals intelligence during the Cold War usefully illuminated the lower levels of the Soviet system, and shows GCHQ’s operational importance to the conduct of colonial and postcolonial conflicts, including the 1982 Falklands War.