Hitchcock did much work on music of the early Baroque in France and Italy, especially on Marc-Antoine Charpentier. He also made important contributions to the understanding of musical traditions in America, both popular and cultivated, and his text in this field is a standard reference work. In addition to Charles Ives, he focused particular attention on contemporary American composers including Virgil Thomson, John Cage, and Henry Cowell. He was the co-editor of the ''New Grove Dictionary of American Music'' and a consultant for American music for ''The New Grove Dictionary of Music and Musicians''.