Human intelligence (intelligence gathering)
Human intelligence is intelligence-gathering by means of human sources and interpersonal communication. It is distinct from more technical intelligence-gathering disciplines, such as signals intelligence (SIGINT), imagery intelligence (IMINT), and measurement and signature intelligence (MASINT). HUMINT can be conducted in a variety of ways, including via espionage, reconnaissance, interrogation, witness interviews, or torture. Although associated with military and intelligence agencies, HUMINT can also apply in various civilian sectors such as law enforcement.
A U.S. Marine asking a local woman about weapons in Fallujah during the Iraq War
Signals intelligence (SIGINT) is the act and field of intelligence-gathering by interception of signals, whether communications between people or from electronic signals not directly used in communication. Signals intelligence is a subset of intelligence collection management. As classified and sensitive information is usually encrypted, signals intelligence may necessarily involve cryptanalysis. Traffic analysis—the study of who is signaling to whom and in what quantity—is also used to integrate information, and it may complement cryptanalysis.
RAF Menwith Hill, a large site in the United Kingdom, part of ECHELON and the UKUSA Agreement in 2005
A German message intercepted by the British during World War II, signaling Germany's unconditional surrender
Zimmermann Telegram, as decoded by Room 40 in 1917
A Mark 2 Colossus computer. The ten Colossi were the world's first programmable electronic computers, and were built to break the German codes.