The technique of keeping a covert watch on individuals and premises. Surveillance may be categorized as observation of suspects, known as physical surveillance, and the monitoringof conversations, referred to as technical surveillance. Physical surveillance may require the deployment of teams of experts, some in vehicles or aircraft, trained to report on the movements of a person without arousing his or her suspicions and to identify attempts at countersurveillance even when they are disguised.
During the Cold War, surveillance techniques grew increasingly sophisticated and included the use of specially developed technology to assist in monitoring the movement of targets. In Moscow this included the application of spy dust, while in London and Washington, D.C., tracking equipment was retrofitted into vehicles used by adversaries.