Guido of Arezzo or Guido Aretinus or Guido da Arezzo or Guido Monaco or Guido D'Arezzo (991/992–after 1033) was a music theorist of the Medieval era. He is regarded as the inventor of modern musical notation (staff notation) that replaced neumatic notation; his text, the Micrologus, was the second-most-widely distributed treatise on music in the Middle Ages (after the writings of Boethius).
Guido was a monk of the Benedictine order from the Italian city-state of Arezzo. Recent research has dated his Micrologus to 1025 or 1026; since Guido stated in a letter that he was thirty-four when he wrote it, his birthdate is presumed to be around 991 or 992. His early career was spent at the monastery of Pomposa, on the Adriatic coast near Ferrara. While there, he noted the difficulty that singers had in remembering Gregorian chants. He came up with a method for teaching the singers to learn chants in a short time, and quickly became famous throughout north Italy. However, he attracted the hostility of the other monks at the abbey, prompting him to move to Arezzo, a town which had no abbey, but which did have a large group of cathedral singers, whose training the Bishop Tedald invited him to conduct.
While at Arezzo, he developed new technologies for teaching, such as staff notation and solfeggio (the progenitor of the "do-re-mi" scale, whose syllables are taken from the initial syllables of each of the first six musical phrases of the first stanza of the hymn, Ut queant laxis). This may have been based on his earlier work at Pomposa, but the antiphoner that he wrote there is lost. Guido is also credited with the invention of the Guidonian hand, a widely used mnemonic system where note names are mapped to parts of the human hand. The Micrologus, written at the cathedral at Arezzo and dedicated to Tedald, contains Guido's teaching method as it had developed by that time. Soon it had attracted the attention of Pope John XIX, who invited Guido to Rome. Most likely he went there in 1028, but he soon returned to Arezzo, due to his poor health. Nothing is known of him after this time, except that his lost antiphoner was probably completed in 1030.
Guido of Arezzo is also the namesake of GUIDO music notation, a format for computerized representation of musical scores.