Karl Johan Schuster (born 1 February 1985), known professionally as Shellback, is a Swedish songwriter, record producer and musician. 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 regularly collaborates with songwriter Max Martin, and together they have produced, written and/or co-written songs for P!nk ("So What", "Raise Your Glass", "Fuckin' Perfect"), Taylor Swift ("We Are Never Ever Getting Back Together", "I Knew You Were Trouble", "22", "Shake It Off", "Blank Space", "Style", "Bad Blood"), Adam Lambert ("Whataya Want from Me", "If I Had You"), Britney Spears ("If U Seek Amy", "3", "I Wanna Go"), Usher ("DJ Got Us Fallin' in Love", "Scream"), Avril Lavigne ("What the Hell", "Wish You Were Here"), Ariana Grande ("Problem"), and Maroon 5 ("One More Night"). Shellback has collaborated with fellow producer Benny Blanco for "Payphone" and "Moves like Jagger" by Maroon 5 and "She Doesn't Mind" by Sean Paul. He has also produced songs by himself, including "Want U Back" by Cher Lloyd and "Animals" and "This Summer's Gonna Hurt" by Maroon 5.