Anaerobic digestion is a biochemical process where complex organic matter such as carbohydrates, proteins, and lipids degrade in the absence of oxygen and are converted into methane and carbon dioxide by the action of different groups of microorganisms. It is a sustainable waste management technology, which reduces and stabilizes organic wastes, recycles its nutrient and water content, while producing energy. Biogas reduces the demand for fossil fuels, since it can be used for the production of electric power and heat, or converted into vehicle fuel. Currently, methane production via anaerobic digestion is a steadily increasing industry in Europe and all over the world. This chapter focuses on the anaerobic digestion process and the parameters affecting its performance. It also describes briefly the current technologies for anaerobic digestion. Finally, since the degradation of organic material requires a synchronized action of different groups of microorganisms with different metabolic capabilities, this chapter also presents recent developments in molecular biology techniques as valuable tools to obtain in-depth understanding about this complex microbiological system.