In music theory, prolongation is the process in tonal music through which a pitch, interval, or consonant triad is considered to govern spans of music when not physically sounding. It is a central principle in the music-analytic methodology of Schenkerian analysis, conceived by Austrian theorist Heinrich Schenker. The English term usually translates Schenker's Auskomponierung (better translated as "composing out" or "elaboration"). According to Fred Lerdahl, "The term 'prolongation' [...] usually means 'composing out' (Schenker's own intention for the term is open to debate)."

Prolongation can be thought of as a way of generating musical content through the linear elaboration of simple and basic tonal structures with progressively increasing detail and sophistication, and thus analysis consists of a reduction from detail to structure. Important to the operation of prolongation is the hierarchical differentiation of pitches within a passage of tonal music. Typically, the note or harmony of highest hierarchical significance is the tonic, and this is said to be "prolonged" across durations of music that may feature many other different harmonies. (However, in principle any other type of consonant chord, pitch, or harmonic function can be prolonged within tonal music.) "In chord prolongation, one chord governs a prolongation of various chords; these different chords are subordinated to that one chord which they help to express and prolong."

A pitch is located in a pitch class, a pitch class is located within a chord, a chord is located in a key region, a key is located in pitch space including the circle of fifths and their relative minors. A rhythmic event is located within the meter which is located within the form. Thus "reductions" are often made at different levels excluding the prolongational from the structural events; these may express the relationships through time-reductions or prologational reductions (which may be Urlinien or tree diagrams).