"Fluorescent Adolescent" is a ballad about getting older and finding out that life isn't as fun anymore. It describes a woman who dreams of her youth while her current life is unsatisfying and boring. The song peaked at number five on the UK Singles Chart and was the 83rd best-selling song of 2007 in the United Kingdom. "Fluorescent Adolescent" is one of the band's most popular songs, and is frequently performed as a closer to their concerts.
On downloads alone, "Fluorescent Adolescent" charted in the UK Singles Chart at number 55 for the week commencing 25 June 2007. The song charted at number five upon release of the single on CD for the week commencing 16 July 2007. On 27 June 2007, "Fluorescent Adolescent" began receiving significant BBC Radio 1 airplay as the first song on "The A List" Playlist. The songs three B-sides—"Plastic Tramp", "The Bakery" and "Too Much to Ask"—appeared on the UK Singles Chart the week of the single's physical release at #153, #161 and #178, respectively.
The music video is based around a fight between a group of clowns and a group of non-clowns at an abandoned warehouse. The presumed leaders of the two groups have a flashback showing that they were friends when they were young children. During the flashback, photos of the band as young children are shown. Throughout the fight, each group leader attempts to kill the other. As the music video climaxes, the non-clown leader tries to run over the clown leader. However the clown leader notices that the non-clown leader's car (with the non-clown leader inside) has been leaking fuel and drops his cigarette lighter. The car quickly catches fire and the clown leader looks on as his former friend burns and the music video ends.
The video was loosely inspired by the poem "Out of Control Circus" by John Cooper Clarke. Turner says he's a fan and gets lyrical inspiration from Clarke. The poem is also printed inside the CD release of "Fluorescent Adolescent." The lines such as "out of control fairground" and "homicidal clowns" support this. The video was preceded by a warning of violent images. In an MTV interview, Turner stated that the music video "is the best thing we've ever done" while Jamie Cook hailed it as "...probably one of the best videos in the last, like, 10 years".
During a live performance of the song on Friday Night with Jonathan Ross, the band played the song dressed as clowns, with Alex Turner dressing with the same make-up and clothing as the main clown in the video.