"20th Century Boy" is a song by T. Rex, written by Marc Bolan. It was released as a single in 1973 and reached No. 3 in the UK Singles Chart. The song was not featured on an original studio album but was included as a bonus track on a reissue of 1973 album Tanx.

It later returned to the UK Top 20 in 1991, peaking at No. 13, fourteen years after Bolan's death, when it was used in a Chris Hartwill-directed commercial for Levi's starring Brad Pitt. It plays a key role on the science fiction-mystery manga 20th Century Boys.

20th Century Boy was recorded on 3 December 1972 in Toshiba Recording Studios in Tokyo, Japan at a session that ran between 15:00 and 1:30.

Backing vocals, hand claps, acoustic guitar and saxophones were recorded in England when T. Rex returned to the country after their tour.

The single version of the track fades out at three minutes thirty nine seconds, however the multi track master reveals that the song ended in nearly a full three minutes worth of jamming. A rough mix of the full length version can be found on the Bump 'n' Grind compilation.

20th Century Boy's lyrics are, according to Marc Bolan, based on quotes taken from notable celebrities such as Muhammad Ali. This can be seen through the inclusion of the line "sting like a bee", which is taken from one of Ali's 1969 speeches.

Although the lyrical content of a lot of Marc Bolan's songs is ambiguous, analysis of the multi track recordings of 20th Century Boy reveals the first line of the song to be "Friends say it's fine, friends say it's good/Everybody says it's just like Robin Hood," and not the often misquoted "...just like rock 'n' roll."

"Free Angel" was recorded during the first session for the Tanx album, between 1 and 4 August 1972. The single was mixed for release at Air Studios on 16 December 1972.

English hard rock band Def Leppard covered the song on their 2006 album, Yeah!, which features cover versions of 1970s rock hits. It was released as the third and final single from the album, on 21 August 2006. The band used the song extensively as promotion for including two TV appearances and a regular rotation in the setlist of their 2006 Yeah! Tour. Def Leppard performed "20th Century Boy" on The Tonight Show with Jay Leno on 23 May 2006 two days before performing the song with Queen's Brian May on VH1 Rock Honors broadcast on 31 May.

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