Helloween is a German power metal band founded in 1984 in Hamburg, Northern Germany. The band is a pioneering force in the power metal genre, and their second and third studio albums, Keeper of the Seven Keys Part I and Part II, are considered masterpieces of the genre. Twelve musicians have been a part of the band's line-up in its history, which has consisted of singer Andi Deris, guitarists Michael Weikath and Sascha Gerstner, bass guitarist Markus Grosskopf, and drummer Daniel Löble since 2005.

Helloween was formed in 1984 by members of bands Iron Fist, Gentry, and Powerfool. Its first line-up consisted of Weikath, Grosskopf, singer and guitarist Kai Hansen, and drummer Ingo Schwichtenberg. After the release of a self-titled EP and a first album in 1985, it expanded into a quintet when singer Michael Kiske joined, with Hansen only acting as guitarist. Under this line-up, they released the Keeper of the Seven Keys albums (1987 and 1988), which established Helloween as a notable heavy metal band and led to the creation of the power metal subgenre. However, Hansen left the band quickly after the release of Part II and was replaced by Roland Grapow. He then created the band Gamma Ray.

Helloween's first two albums without Hansen, Pink Bubbles Go Ape (1991) and Chameleon (1993), were commercial and critical failures, which created tensions between band members and led to the firing of Schwichtenberg and the departure of Kiske. They were replaced by Uli Kusch and Andi Deris respectively. The albums released under this line-up, consisting of Master of the Rings (1994), The Time of the Oath (1996), Better Than Raw (1998) and The Dark Ride (2000), received a warm reception from both fans and critics, gradually establishing Helloween as a successful band again.

However, tensions following the release of The Dark Ride led to the dismissals of both Grapow and Kusch, who then went on to found Masterplan together. Grapow was replaced by Gerstner, but the attempts to find a new drummer were chaotic and no band member was credited for drums on the album Rabbit Don't Come Easy (2003). The band finally found a stable drummer in Löble in 2005. Under this new and current line-up, the longest in its history, the band has released four successful studio albums: Keeper of the Seven Keys: The Legacy (2005), Gambling with the Devil (2007), 7 Sinners (2010), and Straight Out of Hell (2013), with all members except Löble contributing to songwriting. Their fifteenth studio album, My God-Given Right, was released on 29 May 2015.

Since its creation, Helloween has released fifteen studio albums, three live albums, three EPs, and twenty-seven singles, and has sold more than eight million records worldwide.