Morse code is a telecommunications method which encodes text characters as standardized sequences of two different signal durations, called dots and dashes, or dits and dahs. Morse code is named after Samuel Morse, one of several developers of the code system. Morse's preliminary proposal for a telegraph code was replaced by an alphabet-based code developed by Alfred Vail, the engineer working with Morse; it was Vail's version that was used for commercial telegraphy in North America. Friedrich Gerke was another substantial developer; he simplified Vail's code to produce the code adopted in Europe, and most of the alphabetic part of the current international (ITU) "Morse" is copied from Gerke's revision.

The ITU International Morse code encodes the 26 basic Latin letters A to Z, one accented Latin letter (É), the Indo-Arabic numerals 0 to 9, and a small set of punctuation and messaging procedural signals (prosigns). There is no distinction between upper and lower case letters. Each Morse code symbol is formed by a sequence of dits and dahs. The dit duration can vary for signal clarity and operator skill, but for any one message, once the rhythm is established, a half-beat is the basic unit of time measurement in Morse code. The duration of a dah is three times the duration of a dit (although some telegraphers deliberately exaggerate the length of a dah for clearer signalling). Each dit or dah within an encoded character is followed by a period of signal absence, called a space, equal to the dit duration. The letters of a word are separated by a space of duration equal to three dits, and words are separated by a space equal to seven dits.

Morse code can be memorized and sent in a form perceptible to the human senses, e.g. via sound waves or visible light, such that it can be directly interpreted by persons trained in the skill. Morse code is usually transmitted by on-off keying of an information-carrying medium such as electric current, radio waves, visible light, or sound waves. The current or wave is present during the time period of the dit or dah and absent during the time between dits and dahs.

Since many natural languages use more than the 26 letters of the Latin alphabet, Morse alphabets have been developed for those languages, largely by transliteration of existing codes.

To increase the efficiency of transmission, Vail designed the original alphabetic code so that the duration of each symbol was approximately inverse to the frequency that the character it represents occurs in typical English text. Unfortunately, in the course of the revisions that led to the current, international ITU Morse code, the assignment of codes to characters in a few cases became non-optimal, although many encodings are: For instance, the most common letter in English, the letter E, has the shortest code – a single dit. Because the Morse code elements are specified relatively, by proportion, rather than by a fixed duration, the code is usually sent at the highest rate that the receiver is capable of decoding.