The book presents the complex realm of anaerobes and methanogens, which are microorganisms specialized in surviving in environments with very little or no oxygen. Anaerobes and methanogens play a vital role in many ecosystems ranging from the deep oceans to the human gut. The book describes the processes neighboring anaerobes and methanogens undergo to survive in environments, low to no oxygen, through their metabolic processes, genetic capabilities, and physiology. The book elaborates on the role of anaerobes in biogeochemical cycles, human health, and industrial processes. If it is biogas production or degradation of organic matter, anaerobic and methanogenic microorganisms play an important role in the ecosystem within our planet. The book shows a valuable perspective on anaerobes and methanogens and their important role in environments that are vital for life on Earth.