In music, an arrangement is a musical reconceptualization of a previously composed work. It may differ from the original work by means of reharmonization, melodic paraphrasing, orchestration, or development of the formal structure. Arranging differs from orchestration in that the latter process is limited to the assignment of notes to instruments for performance by an orchestra, concert band, or other musical ensemble. Arranging "involves adding compositional techniques, such as new thematic material for introductions, transitions, or modulations, and endings... Arranging is the art of giving an existing melody musical variety".
Arrangement and transcriptions of classical and serious music go back to the early history of this genre. In particular, music written for the piano has frequently undergone this treatment. Pictures at an Exhibition, a suite of ten piano pieces by Modest Mussorgsky, has been arranged over twenty times, notably by Maurice Ravel.
Due to his lack of expertise in orchestration, the American composer George Gershwin had his Rhapsody in Blue arranged and orchestrated by Ferde Grofé.