The Butterfly Effect is the fourth studio album by Portuguese gothic metal band Moonspell, released in 1999, inspired by chaos theory. The album is characterised by a more experimental approach than their previous material, incorporating many elements of electronic music and industrial metal. The music is composed primarily by the band’s keyboardist, Pedro Paixão.

The beginning of "Tired" samples Mozart's Requiem. The length for "K" is actually 3:40. The song is followed by a 9-minute hidden track.

All lyrics written by Fernando Ribeiro.

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