Achille-Claude Debussy (August 22, 1862 – March 25, 1918) was a French composer. He was one of the most important figures in music at the turn of the last century; his music represents the transition from late-romantic music to 20th century modernist music.

Debussy's music was completely unorthodox for its time. His pieces are often characterized by time signatures and rhythmic passages that evoke a feeling of flow rather than adhering to a rigid sense of time. His most dramatic contribution was his disregard for traditional ideas of chord structure and tonality. Elements of the Medieval modes are evident in his music, as well as frequent use of the whole tone scale to create a "dream-like" fluidity to his melodies. Some have also speculated that his more avant garde uses of harmony and rhythm stem partly from his interest in ethnic music, such as the music of Java, which he encountered as a young composer. An equally important contribution is his introduction of the whole-tone scale, in which 6 notes are spread evenly across an octave.

Debussy's impact was far reaching. His free use of harmony, that often disregarded altogether the major/minor chord system based on thirds, has been cited as an influence on the rise of jazz music later in the 20th century. His piano music revitalised the medium. Debussy re-defined the potential of a very familiar instrument, and extended the range of compositional piano sonorities.