Heavy Metal, or Metal, is loud and fast Blues/Acid Rock without the Blues. It is aggressive, menacing Hard Rock. Though Metal is often crass and juvenile, its high speed riffage and soloing is often technically demanding on its musicians. Metal has subdivided and mutated so often during its almost 30 years of existence that it is largely an umbrella term now. Bands whose sounds were influential, (or derivative,) and big enough to typify the entire genre are AC/DC, Black Sabbath, Iron Maiden, and Metallica.