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Sustain is a parameter of musical sound in time. As its name implies, it denotes the period of time during which the sound is sustained before it becomes inaudible. Furthermore, sustain is the third of the four segments in an ADSR envelope. The sustain portion of the envelope begins when the attack and decay portions have run their course, and continues until the key is released. The sustain control is used to determine the level at which the envelope will remain. While the attack, decay, and release controls are rate or time controls, the sustain control is a level control.

Instruments with the shortest sustain are usually drums; a drum beat begins to fade almost instantly. Instruments with the highest possible sustain often involve the passage of air, such as brass or the pipe organ, which have theoretically unlimited sustain.

Within a family of instruments, sustain is affected by a variety of factors. For example, sustain in guitars is determined by factors including body construction (hollow versus solid), body woods, the placing of strings (through the body or atop the body), and pickup design.

In electric guitars and pianos, dedicated pedals are available that prolong the sustain-phase of a tone. Its counterpart in synthesizers is the S (for sustain) in the ADSR envelope.

The sustain pedal is the electronic equivalent of a piano's damper pedal. In most synthesizers, the sustain pedal latches the envelopes of any currently playing or subsequently played notes at their sustain levels, even if the keys are lifted.

There are musical devices used to increase sustain known as Sustainers (such as the Fernandes Guitars patented sustainer system). Sustainers can be used to produce infinite sustain.

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