Karl Johan Schuster, known professionally as Shellback, is a Swedish musician, beat creator, record producer and songwriter.



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.



Schuster was born and raised in Karlshamn, Sweden. He started out as a drummer in local indie rock bands in Karlshamn, as well as recording all instruments on the 2003 Meriah demo ...Turn Him Your Other Cheek (a strictly black metal recording), before joining the hardcore/metal scene as a vocalist for Blinded Colony in 2004, with whom he recorded 2005's Promo Demo and 2006's Bedtime Prayers, drawing comparisons to fellow Swedish melodic death metal band In Flames, before leaving in 2007. In 2005 he also performed as a guest musician on the Faith album Sorg.



At the age of 16, Schuster met Max Martin through their mutual friend Julius. By that time, according to an interview in the Swedish music magazine Stim-magasinet, Schuster's taste in music was "too cool for school" and he had no interest in pop music whatsoever. Julius kept sending Schuster's indie rock/death metal demos to Martin, who became curious about what it would sound like if Schuster would make pop music. So in 2006 Martin invited Schuster to his studio in Stockholm to record a demo with him, and since 2007 Schuster has been signed to Martin's production company Maratone.



In 2015, Shellback collaborated with Swedish hardcore punk band Refused, as he is a both a fan of the band's music and friends with the band's drummer David Sandstrom. Sandstrom sent him the versions of the songs "Elektra" and "366" that Refused had recorded with producer Nick Launay, and Shellback replied with his own arrangements of the two songs. Refused frontman Dennis Lyxzen explained in June 2015 that Shellback's versions were "far more Refused than our versions" and "were better" than the Launay-produced versions. The two songs appear on Refused's comeback album Freedom—"Elektra" was released as a single on April 27, 2015—which was released on the label Epitaph Records on June 26, 2015.