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Telegraph (from Greek: tele τῆλε "at a distance", and graphein γράφειν "to write") is the long-distance transmission of messages without the physical exchange of an object bearing the message. Thus semaphore is a method of telegraphy whereas pigeon post is not.

Telegraphy requires that the method used for encoding the message be known to both sender and receiver. Such methods are designed according to the limits of the signalling medium used. The use of smoke signals, beacons, reflected light signals, and flag semaphore signals are early examples. In the 19th century, the harnessing of electricity brought about the means to transmit signals via electrical telegraph. The advent of radio in the early 1900s brought about radiotelegraphy and other forms of wireless telegraphy. In the Internet age, telegraphic means developed greatly in sophistication and ease of use, with natural language interfaces that hide the underlying code, allowing such technologies as electronic mail and instant messaging.

An electrical telegraph was independently developed and patented in the United States in 1837 by Samuel Morse. His assistant, Alfred Vail, developed the Morse code signalling alphabet with Morse. The first telegram in the United States was sent by Morse on 11 January 1838, across two miles (3 km) of wire at Speedwell Ironworks near Morristown, New Jersey. On 24 May 1844, he sent the message "WHAT HATH GOD WROUGHT" from the Old Supreme Court Chamber in the Capitol in Washington to the old Mt. Clare Depot in Baltimore. This message (quoting Numbers 23:23) was chosen by Annie Ellsworth of Lafayette, Indiana,[15] the daughter of Patent Commissioner Henry Leavitt Ellsworth. The message was all capital letters because the original Morse code alphabet had no question mark or lower case.

The Morse/Vail telegraph was quickly deployed in the following two decades; the overland telegraph connected the west coast of the continent to the east coast by 24 October 1861, bringing an end to the Pony Express.