"40", also known as "40 (How Long)", is the tenth and final track from U2's 1983 album, War. The song is noted for its live performances, often involving the audience singing along for minutes after the band has left the stage. The lyrics are a modification of the Bible's .

The song was released as a commercial single only in Germany, simply to promote U2's appearance at the Loreley Festival in 1983. The single was released on a 7" gramophone record with a B-side of the album version of "Two Hearts Beat as One". Since its live debut on February 26, 1983, in Dundee, "40" has been a staple of U2's live concerts, having been performed almost 400 times.

The song was recorded right at the end of the recording sessions for War. Bassist Adam Clayton had already left the studio, and the three remaining band members decided they didn't have a good song to end the album.Bono, The Edge, and Larry Mullen, Jr. quickly recorded the song with The Edge switching off to both the electric and bass guitar. Bono called the song "40" as he based the lyrics on .

When we were making our third record, the War LP, we were being thrown out of the studio by the studio manager because we had overrun or something and we had one more song to do. We wrote this song in about ten minutes, we recorded it in about ten minutes, we mixed it in about ten minutes and we played it, then, for another ten minutes and that's nothing to do with why it's called '40'.

The first verse of the song is based on Psalm 40:1-2, and the second verse of the song is based on Psalm 40:2-3. The chorus is loosely based on the first two verses of Psalm 40:3, which reads "He put a new song in my mouth, a song of praise to our God..."

"40" debuted live on February 26, 1983, in Dundee as the final song of the show, and closed every single concert on 1983's War Tour. It became very popular as a concert closer, and between its debut and January 10, 1990, there were roughly only twenty tour concerts that did not feature "40" as the closing song. During live performances, Adam Clayton and The Edge would swap instruments so that Adam played guitar and Edge played bass, and the band members would progressively leave stage, with Bono the first to depart, then Clayton, then Edge, and finally, Larry Mullen, Jr. The crowd would often continue to chant the refrain of "How long...to sing this song?" even after the band had left the stage. Live performances of the song are included on the live album Under a Blood Red Sky and the concert film Live at Red Rocks: Under a Blood Red Sky.

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