The generalized anaerobic digestion model developed by the Task Group of IWA is discussed. The structured model includes multiple steps describing biochemical as well as physico-chemical processes. The biochemical steps include disintegration of a particulate substrate to carbohydrates, proteins and lipids, extracellular hydrolysis of these particulate biopolymers to sugars, amino acids, and long chain fatty acids, respectively, acidogenesis from sugars and amino acids to volatile fatty acids and hydrogen; acetogenesis of long chain - and volatile fatty acids to acetate , and separate methanogenesis steps from acetate and from hydrogen and carbon dioxide. The ADM1 is a powerful tool for predicting both steady state and dynamic behaviour of anaerobic digesters. The extensions and applications of the ADM1 model to simulate industrial scale anaerobic digestion processes are reviewed.