The study of extreme acidophiles, broadly defined as microorganisms that grow optimally at pH values below 3, was initiated by the discovery by Waksman and Joffe in the early 1900s of a bacterium that was able to live in the dilute sulfuric acid it generated by oxidizing elemental sulfur. The number of known acidophiles remained relatively small until the second half of the 20th century, but since then has greatly expanded to include representative of living organisms from within all three domains of life on earth, and notably within many of the major divisions and phyla of Bacteria and Archaea. Environments that are naturally acidic are found throughout the world, and others that are manmade (principally from mining metals and coal) are also widely distributed. These continue to be sites for isolating new species, (and sometimes new genera) which thrive in acidic liquor solutions that contain concentrations of metals and metalloids that are lethal to most life forms. The development and application of molecular techniques and, more recently, next generation sequencing technologies has, as with other areas of biology, revolutionized the study of acidophile microbiology. Not only have these studies provided greater understanding of the diversity of organisms present in extreme acidic environments and aided in the discovery of largely overlooked taxa (such as the ultra-small uncultivated archaea), but have also helped uncover some of the unique adaptations of life forms that live in extremely acidic environments. Thanks to the relatively low biological complexity of these ecosystems, systems-level spatio-temporal studies of model communities have been achieved, laying the foundations for ‘multi-omic’ exploration of other ecosystems. This chapter introduces the subject of acidophile microbiology, tracing its origins to the current status quo, and provides the reader of this textbook with general information which provides a backdrop to the more specific topics described in subsequent chapters. AcidophilicFONDECYTFONDECYT