"40", also known as "40 (How Long)", is a song by Irish rock band U2. It is the tenth and final track on their 1983 album, War. The song is noted for its live performances; guitarist the Edge and bassist Adam Clayton trade instruments during performances of it, and as it was commonly played to end their concerts, the band would leave the stage one-by-one as the audience continued to sing the refrain "How long to sing this song?". The lyrics are a modification of the Bible's Psalm 40.

U2 are an Irish rock band formed in Dublin in 1976. The group consists of Bono, the Edge, Adam Clayton, and Larry Mullen Jr.. Initially rooted in post-punk, U2's musical style has evolved throughout their career, yet has maintained an anthemic quality built on Bono's expressive vocals and the Edge's chiming, effects-based guitar sounds. Bono's lyrics, often embellished with spiritual imagery, focus on personal and sociopolitical themes. Popular for their live performances, the group have staged several elaborate tours over their career.