Acute cholangitis and cholecystitis are frequently encountered in daily practice presenting with abdominal pain, icterus, and fever. Those conditions may develop severe and potentially lethal infection such as sepsis without prompt medical care. As ultrasound (US) is a simple and less invasive modality, it is widely used for emergency patients. According to the Tokyo Guidelines (2013) for the management of acute cholangitis and cholecystitis, US should be performed at the initial consultation for all cases in which acute cholecystitis and/or cholangitis are suspected. Enlarged gallbladder, thickening of the gallbladder wall, gallbladder stones, debris echo, and ultrasonographic Murphy’s sign are the most characteristic US findings of acute cholecystitis. But, there are no direct imaging findings that show evidence of bile duct infection. However, biliary dilatation or its etiology detected by US in the presence of Charcot’s triad and general inflammatory response or cholestasis demonstrated by blood tests can support the diagnosis of acute cholangitis. We explain US findings of acute cholecystitis and cholangitis, and severity assessment for acute cholecystitis. We also highlight the mortal conditions of acute cholecystitis such as torsion of the gallbladder, emphysematous cholecystitis, gangrenous cholecystitis, and perforation of the gallbladder.