The abundance of sulfate-reducing bacteria and archaea (SRBA) is impressive and new isolates are being reported continuously. A few decades ago, only two genera of sulfate-reducing bacteria (SRB) had been identified. As of 2018, 92 genera containing more than 420 species of SRB have been isolated and characterized and there are several species of archaea. This book addresses the development of the research with SRBA and includes historical background of this field. Biochemical characterization of the enzymes, cyhromes and electron carriers involved with dissimilatory sulfate reduction are reviewed and the presence of relevant genes in cultured and uncultured SRBA are assessed using genome analysis. The contributions of transmembrane electron transport complexes as related to cell energetics are discussed. This book highlights the unique cellular and molecular features of the SRBA and discusses the biochemical interactions behind their metabolic capabilities which enable SRBA to grow in extreme environments. Examples are provided to detoxify and alleviate pollution situations, to evaluate mechanisms proposed for corrosion of ferrous metals and to examine the effects of SRB on human and animal hosts.