This prize-winning book, first published in 1991, provides a detailed legal account of the development of the UK Official Secrets Acts 1911-1989. In particular, the Espionage section (s.1) of this criminal law is analysed carefully, illustrated by leading cases of UK spies prosecuted under this section, particularly during the 1980's — including MI5 officer Michael Bettaney and Geoffrey Prime who worked at GCHQ. The author also examines problems of evidence in espionage prosecutions, and the consent of the Attorney-General in cases under the Official Secrets Acts. This book remains the definitive treatise on the UK Official Secrets Acts, especially concerning the espionage provisions.