Lecture : Acute cholecystitis is an acute inflammatory disease of the gallbladder. It must be distinguished from biliary colic, which experiences only severe pain without an inflammatory reaction. None of signs or symptoms is pathognomonic and there were no diagnostic criteria until 2007. The lack of standard criteria for diagnosis and severity assessment is reflected by the wide range of reported mortality rates in the literature, and this lack makes it impossible to provide standardized optimal treatment guidelines for patients. An uniform diagnostic criteria for acute biliary infections were necessary for both research and clinical purposes. Furthermore, the lack of standard criteria for severity assessment makes it impossible to provide standardized optimal treatment guidelines for patients. The severity assessment criteria were first presented throughout the world in 2007 along with diagnostic criteria. As far as I know, four more classification criteria were introduced from German, Italy, and America until now, among which the most widely used is the Tokyo guideline severity grade for acute cholecystitis. In this lecture I will address the diagnosis and severity assessment of the acute cholecystitis in depth.