The divisive line between heavy metal and hard rock is one that is often blurry, and the source of much debate over what exactly is "metal" and what is not. Fans could argue all day long about which album deserves the title of "first heavy metal recording." However, it is generally agreed that the roots of heavy metal go back to the late 1960s and early 70s, when bands like Black Sabbath, Blue Cheer and Deep Purple took the bluesy, psychedelic rock sounds of the day and gave them a darker twist. Led Zeppelin was another very influential player in the early days of metal.
Soon enough, the number of bands experimenting with the heavy metal sound was expanding rapidly. The mid to late seventies saw the debuts of KISS, Van Halen, Motorhead, and many others. Judas Priest especially could be said to have influenced the glam-metal movement of the eighties, when metal really hit the mainstream for the first time. Bands like Motley Crue, Iron Maiden, and Def Leppard were all hugely popular, despite their unnatural affinity for hairspray.
The eighties also brought other important developments in the metal world. Not only did the hair bands reach the mass market; during this time many of the less mainstream varieties of heavy metal began developing. The fast, aggressive thrash metal was dominated in the 80s by four main bands: Metallica, Anthrax, Megadeth, and Slayer. Death metal also got its start in the eighties with the releases of albums by Death and Possessed.