Anaerobic digestion is a biological process, where synergistic actions between bacteria are occurring at four different levels. First, hydrolysis converts a wide range of solid organic materials into sugars and amino acids. Fermenting these materials produces volatile fatty acids (VFAs). Acidogenesis forms hydrogen, carbon dioxide (CO2), and acetate from VFAs. Finally, methanogenesis produces biogas, a mixture of methane, CO2, and numerous trace elements. The key biological issues are determining the most favorable conditions for each process stage and how nonoptimal circumstances affect the process as a whole, and the governing role of hydrogen generation and consumption.