This text provides a close-up look at the ingenious methods by the British used to obtain and analyse secret information and deliver it to operational forces at sea during the wars with France 1793-1815. Drawing on material from a broad range of sources, the author not only sets out the structure of the British intelligence system in this period, centered as it was on the diplomatic network, but also shows how that information was passed to and exploited by Lord Nelson and his contemporaries in a series of case studies, including the famous attack on the Danish fleet at Copenhagen in 1801.