Musical Instrument Digital Interface, or MIDI, is an industry-standard electronic communication protocol that defines each musical note in an electronic musical instrument such as a synthesizer, precisely and concisely, allowing electronic musical instruments and computers to exchange data, or "talk", with each other. MIDI does not transmit audio - it simply transmits digital information about a music performance.

Almost all music recordings today utilize MIDI as a key enabling technology for recording music. In addition, MIDI is also used to control hardware including recording devices as well as live performance equipment such as stage lights and effects pedals. Lately, MIDI has exploded onto the scene with its adoption into mobile phones. MIDI is used to play back the ringtones of MIDI capable phones. MIDI is also used to provide game music in some video games.

The MIDI standard was first proposed by Dave Smith in 1981 in a paper to the Audio Engineering Society and the MIDI Specification 1.0 was published in August 1983.

MIDI allows computers, synthesizers, sound cards and drum machines to control one another, and to exchange system information. Though modern computer sound cards are MIDI-compatible and capable of creating realistic instrument sounds, the fact that sound cards' MIDI synthesizers have historically produced sounds of dubious quality has tarnished the image of a general purpose computer as a MIDI instrument. This despite the fact that the MIDI specification itself has nothing to do with the quality of the sound produced which varies depending on the sound card used.

MIDI is almost directly responsible for bringing an end to the "wall of synthesizers" phenomenon in 1970s-80s rock music concerts, when musical keyboard performers were sometimes hidden behind banks of various instruments. Following the advent of MIDI, many synthesizers were released in rack-mount versions, enabling performers to control multiple instruments from a single keyboard. Another important effect of MIDI has been the development of hardware and computer-based sequencers, which can be used to record, edit and play back performances.

Synchronization of MIDI sequences is made possible by the use of MIDI timecode, an implementation of the SMPTE time code standard using MIDI messages, and MIDI timecode has become the standard for digital music synchronization.

A number of music file formats have been based on the MIDI bytestream. These formats are very compact; often a file of only 10 kilobytes can produce a full minute of music.