Ayreon i/ˈɛriən/ is a musical project by Dutch songwriter, singer, multi-instrumentalist musician and record producer Arjen Anthony Lucassen. Ayreon's music is described as progressive rock, progressive metal and power metal sometimes combined with genres such as folk, electronica, experimental and classical music. The majority of Ayreon's albums are dubbed "rock operas" (or "metal operas") because the albums contain complex storylines featuring a host of characters, usually with each one being represented by a unique vocalist.

Each Ayreon album tells a different story, but all, with the notable exceptions of Actual Fantasy and The Theory of Everything, take place in the same fictional, science fiction universe; additionally, Lucassen's solo album Lost in the New Real is also set in the Ayreon universe. Ayreon's music is characterized by the use of traditional instruments in rock music (guitars, bass guitar, drums, analogue synthesizers, electric organs) mixed with instruments more native to folk and classical music (e.g. mandolins, violins, violas, celli, flutes, sitars and didgeridoo). Lucassen writes the music and the lyrics, sings and plays most of the instruments on all of the Ayreon albums, alongside many guest musicians. His most regular collaborator is drummer Ed Warby.

Due to its particular nature, Ayreon live performances are extremely rare; Several Ayreon songs were included in two live albums by other Lucassen bands: Live on Earth by Star One and Live in the Real World by Stream of Passion, and in September 2015, The Human Equation was performed in its entirety live in a full-scale theatrical production called The Theater Equation, which was later released as a live album and DVD.