Karl Johan Schuster (born February 1, 1985), better known by his stage name Shellback, is a songwriter, record producer and musician from Karlshamn, a small town in Southern Sweden. Together with Max Martin, he wrote "So What" and "Raise Your Glass" by Pink, "If U Seek Amy" and "3" by Britney Spears, "DJ Got Us Fallin' in Love" by Usher featuring Pitbull, and "Whataya Want from Me" by Adam Lambert. Also with Martin he co-wrote the Maroon 5 single One More Night.