Karl Johan Schuster (Swedish: [ˈjûːan ˈɧʏ̌sːtɛr]; born 1 February 1985), known professionally as Shellback, is a Swedish record producer, songwriter, and multi-instrumentalist. Shellback was listed as the No. 1 producer of 2012 on Billboard magazine's year end chart, and he also topped the list of their "Top 10 Songwriters Airplay Chart" the same year; he has won four Grammy Awards.
He regularly collaborates with songwriter Max Martin, and together they have produced, written, or co-written songs for P!nk, Taylor Swift, Adam Lambert, Britney Spears, Usher, Avril Lavigne, Ariana Grande, Adele, and Maroon 5. Shellback has also collaborated with fellow producer Benny Blanco and has produced songs by himself, including "Want U Back" by Cher Lloyd and "Animals" by Maroon 5.