The task of adapting a composition for musical instruments/ensembles, called arranging or orchestrating, may be undertaken by the composer or separately by an arranger based on the composer’s core composition. A composition may have multiple arrangements based on such factors as intended audience type and breadth, musical genre or stylistic treatment, recorded or live performance considerations, available musicians and instruments, commercial goals and economic constraints.

Based on such factors, composers or arrangers must decide upon the instrumentation of the original work. Today, the contemporary composer can virtually write for almost any combination of instruments. Some common group settings include music for Full Orchestra (consisting of just about every instrument group), Wind Ensemble (or Concert Band, which consists of larger sections and greater diversity of wind, brass and percussion instruments than are usually found in the orchestra), or a chamber group (a small number of instruments, but at least two). The composer may also choose to write for only one instrument, in which case this is called a solo.

Composers are not limited to writing only for instruments, they may also decide to write for voice (including choral works, operas, and musicals) or percussion instruments or electronic instruments. Alternatively, as is the case with musique concrete, the composer can work with many sounds often not associated with the creation of music, such as typewriters, sirens, and so forth.