Acute cholecystitis represents the most common cause of upper quadrant abdominal pain and the leading cause of emergency cholecystectomy. In most cases, it is due to cholelithiasis (hence is it defined as acute calculous cholecystitis). In a minority of cases, it is due to other causes and is defined as acute acalculous cholecystitis. Acute cholangitis is less common than cholecystitis, usually caused by a biliary tree obstruction; it is a pathological entity that must be well known as it is a condition characterized by high mortality and which requires immediate intervention. In this chapter, we describe the main imaging characteristics, with particular attention to CT, of these pathologies and their major complications.