Telex is an international message-transfer service for subscribers using teleprinters. A subscriber attaches its Telex-compatible teleprinter to a Telex line, then dials (much like atelephone call) the fellow subscriber to whom the message is being sent. The message travels as a low-bit-rate electrical signal. The recipient's Telex-compatible teleprinter converts the signal, then prints out the message.

Telex originated in Germany in the early 1930s and soon expanded to other European countries. In the United States, American Telephone & Telegraph Company (AT&T) started a manually-switched teletypewriter exchange service called TWX in 1931. Police departments were one of TWX's prime early users. TWX became automatically switched in 1962 and began serving Canada in 1963. Western Union Telegraph Company initiated its Telex system in the United States in 1962, registering the name as a trademark. (In Europe it had been a generic term.)

After Western Union acquired the TWX network in 1970, Western Union linked Telex and TWX, which became respectively known as Telex I and Telex II. Because Telex I teleprinters use the Baudot code and Telex II (TWX) teleprinters use the ASCII code, and transmit at different speeds, exchange of messages between the two systems is somewhat delayed while a processing computer does the necessary conversions. Other companies in the United States, such as MCI and ITT, also provide switched teleprinter exchange services.

By 1990 more than 190 countries used Telex, and in that year, AT&T bought Western Union's Telex network. Sometimes called the first electronic mail service, Telex was, at one time, the most-used written telecommunication system in the business world. Though Telex is still available as of 1998, the widespread use of fax machines and computers, especially electronic mail and the Internet, have lead to Telex's severe decline in popularity.