Numbers stations (or number stations) are shortwave radio stations of uncertain origin. In the 1950s, Time magazine reported that the numbers stations first appeared shortly after the war (WW2) and were using a format that had been used to send weather data during that war.

Numbers stations generally broadcast artificially generated voices reading streams of numbers, words, letters (sometimes using a spelling alphabet), tunes or Morse code. They are in a wide variety of languages and the voices are usually female, although sometimes men’s or children’s voices are used.

Evidence supports popular assumptions that the broadcasts are used to send messages to spies. This usage has not been publicly acknowledged by any government that may operate a numbers station, although two QSLs have been received from numbers stations by SWLers that had sent them reception reports.

In 2001, the United States tried the Cuban Five on the charge of spying for Cuba. That group had received and decoded messages that had been broadcast from a Cuban numbers station. Also in 2001, Ana Belen Montes, a senior US Defense Intelligence Agency analyst, was arrested and charged with espionage. The federal prosecutors stated: “Montes communicated with the Cuban Intelligence Service through encrypted messages and received her instructions through encrypted shortwave transmissions from Cuba”. In 2006, Carlos Alvarez and his wife, Elsa, were arrested and charged with espionage. The U.S. District Court Florida stated: “defendants would receive assignments via shortwave radio transmissions”.

In June 2009, the United States similarly charged Walter Kendall Myers with conspiracy to spy for Cuba and receiving and decoding messages broadcast from a numbers station operated by the Cuban Intelligence Service to further that conspiracy.

It has been reported that the United States uses numbers stations to communicate encoded information to persons in other countries. The state department operated several stations, such as KKN50, that broadcast similar “numbers” messages.

Numbers stations appear and disappear over time (although some follow regular schedules), and their overall activity has decreased since the early 1990s.