We know you know that Metallica, Slayer, Megadeth and Anthrax are considered the "Big Four" of thrash metal, but who were the pioneers of the genre that inspired them in the first place?

It's pretty clear that thrash is defined by aggression and very fast instrumentation. But when you compare those four aforementioned groups, in addition to all the other non-"Big Four" thrash groups such as Overkill, Testament and Exodus, there are actually a lot of differences in their sound and style. They would all go on to influence metal bands in very different subgenres. With that, where was thrash derived from? Which bands were responsible for the arrival of this new category of metal?

The two main components that would contribute to the rise of thrash were the New Wave of British Heavy Metal groups — such as Iron Maiden and Judas Priest — and the punk groups — the Ramones, the Misfits — of the mid to late 1970s. The rapid playing, the attitude and the anti-establishment messages of both groups played a pivotal role in the emergence of thrash.

There are plenty of other bands that don't quite get the same recognition for their influence on the genre, so we've compiled a list of names that deserve to be noted. Scroll through the gallery below to see 15 bands who are considered pioneers of thrash. 

Top 50 Thrash Metal Albums of All Time