Björn Kristian Ulvaeus (born 25 April 1945) is a Swedish musician, singer, songwriter, and producer best known as a member of the musical group ABBA. He is also the co-composer of the musicals Chess, Kristina från Duvemåla, and Mamma Mia! He co-produced the films Mamma Mia! and Mamma Mia! Here We Go Again with fellow ABBA member and close friend Benny Andersson. He is the oldest member of the group.

Björn Kristian Ulvaeus was born in Gothenburg on 25 April 1945. In 1951, he moved with his family to Västervik, Kalmar County. His parents were Aina Eliza Viktoria (née Bengtsson; 1909–2005) and Erik Gunnar Ulvaeus (1912–1999). Ulvaeus has one sister, Eva Margareta (born 1948). Ulvaeus studied business and law at Lund University after undertaking his military service, alongside comedian Magnus Holmström.

Before gaining international recognition with ABBA, Ulvaeus was a member of the Swedish folk-schlager band Hootenanny Singers, known earlier as the "West Bay Singers", who had an enormous following in Scandinavia. While on the road in southern Sweden in 1966, they encountered the Hep Stars, and Ulvaeus quickly became friends with the group's keyboard player, Benny Andersson. The two musicians shared a passion for songwriting, and each found a composing partner in the other. On meeting again that summer, they composed their first song together: "Isn't It Easy To Say", a song soon to be recorded by Andersson's group. The two continued teaming up for music, helping out each other's bands in the recording studio, and adding guitar or keyboards respectively to the recordings. In 1968, they composed two songs together: "A Flower in My Garden", recorded by Hep Stars, and their first real hit "Ljuva Sextiotal", for which Stig Anderson wrote lyrics. The latter, a cabarét-style ironic song about the 1960s, was submitted for the 1969 Swedish heats for the Eurovision Song Contest, but was rejected; it was later recorded by diva Brita Borg. Another hit came in 1969 with "Speleman", also recorded by Hep Stars.

While filming a nostalgic schlager special for television in March 1969, Björn met eighteen-year-old future wife and singer-songwriter Agnetha Fältskog.

Björn Ulvaeus continued recording and touring with Hootenanny Singers to great acclaim while working as in-house producer at Polar Record Company (headed by future manager Stig Anderson), with Benny as his new partner. The twosome produced records by other artists and continued writing songs together. Polar artist Arne Lamberts Swedish version of "A Flower in My Garden" ("Fröken Blåklint") was one of Björn & Benny's first in-house productions. In December 1969, they recorded the new song "She's My Kind of Girl", which became their first single as a duo. It was released in March 1970, giving them a minor hit in Sweden and a top-ten hit in Japan two years later.

The Hootenanny Singers entered Svensktoppen, the Swedish radio charts, in 1970 with "Omkring Tiggarn Från Luossa", a cover of an old folk-schlager song. It remained on the charts for 52 consecutive weeks, a record which endured until 1990; the song was produced by Björn and Benny, and had Ulvaeus's solo vocal and Benny's piano. ...

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