An electrical telegraph was a point-to-point text messaging system, used from the 1840s until the late 20th century when it was slowly replaced by other telecommunication systems. At the sending station switches connected a source of current to the telegraph wires. At the receiving station the current activated electromagnets which moved indicators, providing either a visual or audible indication of the text. It was the first electrical telecommunications system and the most widely used of a number of early messaging systems called telegraphs, that were devised to communicate text messages more rapidly than by physical transportation. Prior to the electric telegraph, semaphore systems were used, including beacons, smoke signals, flag semaphore, and optical telegraphs for visual signals to communicate over distances of land.Electrical telegraphy can be considered to be the first example of electrical engineering, and was used by the emerging railway companies to develop train control systems that minimised the chances of trains colliding with each other. This was built around the signalling block system with signal boxes along the line communicating with their neighbouring boxes by telegraphic sounding of single-stroke bells and three-position needle telegraph instruments.

Text telegraphy consisted of two or more geographically separated stations (often called telegraph offices) connected by wires, usually supported overhead on utility poles (originally called telegraph poles). There were many different electrical telegraph systems invented, but the ones that became widespread fit into two broad categories.

The first category consists of needle telegraphs in which a needle pointer is made to move electromagnetically with an electric current from a battery or dynamo passing down the telegraph line. Early systems used multiple needles requiring multiple wires. The first commercial system, and the most widely used needle telegraph, was the Cooke and Wheatstone telegraph, invented in 1837. Early equipment sets used five needles to point to the letter being transmitted, but the cost of installing wires was more economically significant than the cost of training operators so a single-needle system with a code that had to be learned became the norm.

The second category consists of armature systems in which the current activates a telegraph sounder which makes a click. The archetype of this category was the Morse system, invented by Samuel Morse in 1838, using a single wire. At the sending station, an operator would tap on a switch called a telegraph key, spelling out text messages in Morse code. Originally, the armature was intended to make marks on paper tape, but operators learned to interpret the clicks and it was more efficient to write down the message directly. In 1865, the Morse system became the standard for international communication with a modified code developed for German railways. However, some countries continued to use established national systems internally for some time afterwards.

In the 1840s, the electrical telegraph superseded optical telegraph systems (except in France), becoming the standard way to send urgent messages. By the latter half of the century, most developed nations had created commercial telegraph networks with local telegraph offices in most cities and towns, allowing the public to send messages called telegrams addressed to any person in the country, for a fee. Beginning in 1854, submarine telegraph cables allowed for the first rapid communication between continents. Electrical telegraph networks permitted people and commerce to transmit messages across both continents and oceans almost instantly, with widespread social and economic impacts. In the early 20th century the telegraph was slowly replaced by teletype networks.