The best-known element of signals intelligence during the First World War is work against the operational traffic of armies and navies, centring on cryptanalysis and traffic analysis. However, overwhelmingly its largest element during that conflict, and the area where signals intelligence was most frequently used, lay in economic warfare. This instance was perhaps the case in history where communications intelligence worked best without the aid of cryptanalysis. It was the first case where both communications intelligence and powerful and sophisticated modes of analysis were applied to strategy and economic warfare. These issues were fundamental to a central part of the Great War: the blockade. Studies of the blockade routinely mention communications intelligence, but rarely treat it systematically. Thus, a key dimension in the history of the blockade, and signals intelligence, is overlooked. Between 1914 and 1918, British intelligence intercepted and read 80,000,000 telegrams, 25,000,000 radio messages and 630,000,000 postal packets, possibly involving over 1,000,000,000 letters, since business packages routinely carried many messages. These are big numbers. They dwarf any collection of communications, or intelligence, ever known before 1914. These messages were processed in real time and applied effectively to economic warfare. Without this power in collection and assessment, the blockade would have taken a different form.