Often regarded as the group's signature song, it has been described by Rolling Stone as "a seven-and-a-half-minute statement of purpose: rock power, jazz spontaneity, and classical precision harnessed in the service of a common aim" and as an early example of heavy metal.

[1] The lyrics of "21st Century Schizoid Man" were written by Peter Sinfield and consist chiefly of disconnected phrases which present a series of images in a fixed pattern.

[1] The song criticizes the Vietnam War with the lyrics "Politicians' funeral pyre/Innocents raped with napalm fire" and "death seed", which according to Sinfield refers to a "harvest of bad things" created by the use of Agent Orange.

"[2][3] The eerie intro sound was created by woodwind and keyboard player Ian McDonald pressing his forearm onto the keys of the studio's bellows-driven reed organ, causing the instrument to malfunction and "wheeze" unpredictably.

"[8] After a reprise of the main riff and a final verse, the song concludes with an abstract chaotic outro with no discernible meter, inspired by the Duke Ellington Orchestra.

[9] In 2006, Pitchfork ranked "21st Century Schizoid Man" at number 104 on its list of "The 200 Greatest Songs of the 1960s", with Joe Tangari calling it a "seven-minute hellstorm of gonzo guitar, shifting meters, and nasty sax".