Ayreon /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 metal and progressive rock sometimes combined with genres such as folk, power metal, classical and electronica. 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.

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. In addition, flute player Jeroen Goossens has featured on every Ayreon-release since 2004's "The Human Equation".