"Ágætis byrjun" is the second album of the Icelandic post-rock band. It was released on June 12, 1999, in the UK in 2000 and in the United States in 2001. The ten songs on the record were recorded between summer 1998 and spring 1999 with producer Ken Thomas. It was the first album with keyboardist Kjartan Sveinsson and the last with drummer Ágúst Ævar Gunnarsson, who left the band a few months after the album's release

The music is a marked departure from the sound of the previous album, their debut "From". The album's expansive ambient soundscapes have been replaced with Jónsi Birgisson's cello-accompanied guitar work and orchestration, featuring a double string octet among other chamber music elements. The songs, as with almost all Sigur Rós albums, but unlike the follow-up "( )", are almost entirely sung in Icelandic, the native language of the band members. Only the song "Olsen Olsen" is sung by singer Jónsi in the language he invented, "Vonlenska".

With "Ágætis byrjun" Sigur Rós were able to celebrate their breakthrough, both commercially and in terms of reviews. It sold 10,000 copies in Iceland in its first year of release and was certified platinum. It also won several awards and appeared on several critics' lists of the best albums of the 2000s.

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