New Complexity is a term dating from the 1980s, principally applied to composers seeking a "complex, multi-layered interplay of evolutionary processes occurring simultaneously within every dimension of the musical material" (Fox 2001). Usually the most prominent recognizable feature, and often the most difficult for the performer to realize, is the complex rhythmic tuplet notation (sometimes called irrational rhythm) used in the written scores.

Though often atonal, highly abstract, and dissonant in sound, the "New Complexity" is most readily characterized by the use of techniques which require complex musical notation. This includes extended techniques, microtonality, odd tunings, highly disjunct melodic contour, innovative timbres, complex polyrhythms, unconventional instrumentations, abrupt changes in loudness and intensity, and so on.