"Ar Hyd y Nos" ("All through the Night") is a Welsh folksong sung to a tune which was first recorded in Edward Jones' Musical and Poetical Relics of the Welsh Bards (1784). The Welsh lyrics were written by John Ceiriog Hughes. It has been translated into several languages, including English (most famously by Harry Boulton). The song is included in Fantasia on British Sea Songs at the Last Night of the Proms, representing Wales.
The melody was used by John Gay in The Beggar's Opera. It is also used in the hymn "Go My Children With My Blessing."
The song is highly popular with traditional Welsh male voice choirs, and is sung by them at festivals in Wales and around the world.
The song is also sometimes considered a Christmas carol, and as such has been covered by numerous artists on Christmas albums, most recently including Olivia Newton-John and Michael McDonald who performed the song as a duet on Newton-John's 2007 album Christmas Wish.