Piano Phase is a piece of music written in 1967 by the minimalist composer Steve Reich for two pianos. It is his first attempt at applying his "phasing" technique which he had previously used in the tape pieces It's Gonna Rain (1965) and Come Out (1966) to live performance.

Reich's phasing works generally have two identical lines of music, which begin by playing synchronously, but slowly become out of phase with one another when one of them slightly speeds up. Reich had previously applied this technique only to sounds recorded on magnetic tape, but experimenting in his studio, he found it was possible for humans to replicate the effect. In Piano Phase, he has the two pianists begin by playing a rapid twelve note melodic figure over and over again in unison. After a while, one of the pianists begins to play their part slightly faster than the other. When they are playing the second note of the figure at the same time the other pianist is playing the first note, the two pianists play at the same tempo again. They are therefore playing notes at exactly the same time, but they are not the same notes, as they were at the start of the piece. The process is repeated, so that the second pianist plays the third note as the first pianist is playing the first, then the fourth, and so on until the process has gone full circle, and the two pianists are playing in perfect unison again. The second pianist then fades out, leaving the first playing the oriinal 12 note melody. They then seamlessly change to a similar melody made up of 8 notes. the second piano fades in again, only this time playing a different 8 note melody at the same time. the phasing then begins again. after the full eight cycles have gone through, the first pianist fades out, leaving one 8 note melody playing. after a few repetitions, the pianist then takes out the first 2 notes of the melody and the first pianist fades in in unison. they phase through the now four cycles, and finish after returning in unison.

The music is made up, therefore, of nothing more than the results of applying the phasing process to the initial twelve-note melody - as such, it is a piece of process music.

The piece is played by two pianists without breaks at any stage. A typical performance may last around fifteen to twenty minutes. Reich later made a version for two marimbas.

Reich further developed this technique in pieces like Violin Phase (also 1967) and Clapping Music (1972), and it is often used alongside other techniques in later works, such as The Desert Music (1984).