Guido of Arezzo was an Italian music theorist and composer who lived during the 11th century. He is best known for developing a system of musical notation that laid the groundwork for modern Western music. His innovations included the use of staff lines and the introduction of the solfège system, which assigns syllables to notes in a scale. Guido's most significant work, the Micrologus, provided practical guidance on music theory and teaching. His methods greatly influenced the way music was taught and notated in medieval Europe, making him a pivotal figure in the history of music education.