This brochure — prepared by the National Security Agency, Center for Cryptologic History — defines and explains traffic analysis when used in this context, as part of the broader discipline of signals intelligence. It describes the elements of traffic analysis and explains how traffic analysis has been used to produce intelligence information, to aid cryptanalysis, and to support the collection of additional data. It presents examples of intelligence contributions made by traffic analysis during World War I, World War II, and the Cold War through the Vietnam War.