Helloween is a German power metal band founded in 1984 in Hamburg by members of bands Iron Fist, Gentry, Second Hell and Powerfool. The band has at times been called one of the most influential European heavy metal bands of the 1980s. Its first lineup consisted of singer and guitarist Kai Hansen, bassist Markus Grosskopf, guitarist Michael Weikath and drummer Ingo Schwichtenberg. By the time Hansen left Helloween in 1989 to form Gamma Ray, the band had evolved into a five-piece, with Michael Kiske taking over as lead vocalist. Schwichtenberg and Kiske both parted ways with Helloween in 1993; Schwichtenberg died two years later as the result of suicide. Between then and 2016, there had been numerous line-up changes, leaving Grosskopf and Weikath as the only remaining original members. As a septet, their current lineup features all of the surviving members of the Keeper of the Seven Keys lineup (Hansen, Weikath, Kiske and Grosskopf), in addition to the remaining members from the Keeper of the Seven Keys: The Legacy-era (singer Andi Deris, guitarist Sascha Gerstner and drummer Daniel Loble).

Since its inception, Helloween has released 17 studio albums, five live albums, three EPs and 31 singles. The band was honored with 14 gold and six platinum awards and has sold more than 10 million records worldwide. Helloween has been referred to as the "fathers of power metal", as well as one of the so-called "big four" of the genre's early German scene, along with Grave Digger, Rage and Running Wild, and one of the "big four" bands of the entire genre, alongside Blind Guardian, Sabaton, and DragonForce.

The staff of Loudwire named them the 34th-best metal band of all time.

Helloween was formed 1984 in Hamburg, West Germany. Its name was inspired by the film Halloween. The original line-up included Kai Hansen on vocals and rhythm guitar, Michael Weikath on lead guitar, Markus Grosskopf on bass and Ingo Schwichtenberg on drums. That year, the band signed with Noise Records and recorded two songs for a Noise compilation album called Death Metal. The compilation featured the bands Hellhammer, Running Wild and Dark Avenger. The two tracks were "Oernst of Life" by Weikath and Hansen's "Metal Invaders," a faster version of which would appear on the band's first full-length album.

Helloween recorded and released its first record in 1985, a self-titled EP containing five tracks. Also that year, the band released its first full-length album, Walls of Jericho, which helped spawn speed metal. During the following concert tour, Hansen had difficulties singing and playing the guitar at the same time. Hansen's last recording as the band's lead singer was in 1986 on a vinyl EP titled Judas, which contained the song "Judas" and live versions of "Ride the Sky" and "Guardians" recorded at Gelsenkirchen. (The CD edition has the live introduction, but the songs have been replaced with studio versions and crowd noise spliced in.) Following these releases, Helloween started the search for a new vocalist.

Hansen explained in an interview 1999:

"We wanted to go for a lot more intricate stuff on the guitar and more technical. So it was like getting harder and harder for me without having big experience on vocals. I never had lessons or anything and I was not that fixed on the guitar yet so it was getting difficult. On the other hand we thought it would be really cool to have a real front man, a real singer. We heard Queensryche, the first album that they did, and we thought wow that would be cool to have a singer like that who could be very stable on the high soaring notes and have a lot of expression."

The band found an 18-year-old vocalist, Michael Kiske, from a local Hamburg band named Ill Prophecy. Kiske was initially uninterested in them, having heard the more thrashy Walls of Jericho, but after Weikath insisted, he attended one of their sessions and heard some songs they had composed for his voice (songs which would later be featured in their next albums), and he changed his mind.