The chapter discusses the energetics of syntrophic cooperation in methanogenic degradation, a process where complex organic matter is converted to methane and CO₂ in anoxic environments. This process is less exergonic compared to aerobic degradation, releasing only 15% of the energy available in aerobic conditions. The low energy yield forces microorganisms to form highly efficient symbiotic relationships, where the mutual dependence of partner bacteria is crucial for survival. The term "syntrophy" refers to the close cooperation between two metabolically different bacteria, such as fatty acid-oxidizing bacteria and hydrogen-oxidizing methanogens, which depend on each other for substrate degradation. The chapter highlights the importance of maintaining low hydrogen partial pressures to facilitate interspecies hydrogen transfer, a key step in syntrophic processes. It also discusses the energetic challenges and strategies used by bacteria to manage their energy metabolism, including the use of reversed electron transport systems to couple ATP synthesis with proton reduction. Examples of syntrophic associations, such as the conversion of ethanol to acetate and methane by "Methanobacillus omelianskii," are provided to illustrate these concepts. The chapter further explores the degradation of specific substrates, including fatty acids, alcohols, and benzoate, and the role of different bacterial groups in the methanogenic degradation process. It emphasizes the importance of acetate-cleaving methanogens in removing acetate, which is crucial for maintaining the overall energy balance in syntrophic systems. The energetic and biochemical aspects of these processes are detailed, including the involvement of various enzymes and redox reactions. Overall, the chapter provides a comprehensive overview of the energetic and biochemical mechanisms underlying syntrophic cooperation in methanogenic degradation, highlighting the intricate interactions and energy management strategies employed by these microorganisms.The chapter discusses the energetics of syntrophic cooperation in methanogenic degradation, a process where complex organic matter is converted to methane and CO₂ in anoxic environments. This process is less exergonic compared to aerobic degradation, releasing only 15% of the energy available in aerobic conditions. The low energy yield forces microorganisms to form highly efficient symbiotic relationships, where the mutual dependence of partner bacteria is crucial for survival. The term "syntrophy" refers to the close cooperation between two metabolically different bacteria, such as fatty acid-oxidizing bacteria and hydrogen-oxidizing methanogens, which depend on each other for substrate degradation. The chapter highlights the importance of maintaining low hydrogen partial pressures to facilitate interspecies hydrogen transfer, a key step in syntrophic processes. It also discusses the energetic challenges and strategies used by bacteria to manage their energy metabolism, including the use of reversed electron transport systems to couple ATP synthesis with proton reduction. Examples of syntrophic associations, such as the conversion of ethanol to acetate and methane by "Methanobacillus omelianskii," are provided to illustrate these concepts. The chapter further explores the degradation of specific substrates, including fatty acids, alcohols, and benzoate, and the role of different bacterial groups in the methanogenic degradation process. It emphasizes the importance of acetate-cleaving methanogens in removing acetate, which is crucial for maintaining the overall energy balance in syntrophic systems. The energetic and biochemical aspects of these processes are detailed, including the involvement of various enzymes and redox reactions. Overall, the chapter provides a comprehensive overview of the energetic and biochemical mechanisms underlying syntrophic cooperation in methanogenic degradation, highlighting the intricate interactions and energy management strategies employed by these microorganisms.