Metal music has been undergoing a remarkable sonic development. Pioneering releases of the early 1970s by Black Sabbath already contained all essential ingredients of metal’s sonic signature. The growing need for heavier sounds was afforded by a rapidly advancing recording technology, alongside the exploration of production techniques and aesthetics. This chapter traces significant developments in metal music production from the 1970s to the 1990s by looking at key artists, albums and audio professionals to outline how heaviness in recorded form developed in the genre. Many of the analysed engineering practices were adopted, improved and have become standard in contemporary metal production. In this process, production was brought to the fore, making it an increasingly fundamental element of the music, even an art form in itself. This development led to what has become metal’s standard hyper-real aesthetic, which will most likely be pushed further in the ongoing and genre-defining quest for greater heaviness.