Signals intelligence (often contracted to SIGINT) is intelligence-gathering by interception of signals, whether between people ("communications intelligence"—COMINT), whether involving electronic signals not directly used in communication ("electronic intelligence"—ELINT), or combinations of the two. As sensitive information is often encrypted, signals intelligence often involves the use of cryptanalysis. Also, traffic analysis—the study of who is signaling whom and in what quantity—can often produce valuable information, even when the messages themselves cannot be decrypted.

As a means of collecting intelligence, signals intelligence is a subset of intelligence collection management, which, in turn, is a subset of intelligence cycle management.

Intercepting written but encrypted communications, and extracting information, probably did not wait long after the development of writing. A simple encryption system, for example, is the Caesar cipher. Electronic interception appeared as early as 1900, during the Boer Wars. The Boers had captured some British radios, and, since the British were the only people transmitting at the time, no special interpretation of the signals was necessary.