The sonic evolution of heavy metal is the story of the seminal musicians who engineered the metal sound and its numerous interpretations. This chapter illustrates that evolvement, highlighting key bands and performers whilst exemplifying their innovative techniques and concepts. It was a process initiated by Black Sabbath in the early 1970s and subsequently expanded, firstly by the New Wave of British Heavy Metal bands of the later 1970s, and then, more radically, through the emergence of thrash, death metal, black metal and grindcore during the 1980s. Subsequent innovators, from the 1990s onwards, adopted fusion as a way of redefining metal. By incorporating musical elements derived from pop, hip-hop and classical and folk music, they were able to fashion a plethora of imaginative crossovers, including nu metal, folk metal, symphonic metal, industrial metal and dance metal. The chapter concludes by noting the most recent developments in metal where elaborate assimilations of contrasting musical idioms showcase established metal devices in new and unusual contexts.