Gallstones (cholelithiasis) appear on ultrasound as hyperechoic (bright) foci within the gallbladder lumen that cast a clean posterior acoustic shadow. They are mobile — moving with changes in patient position — which helps distinguish them from gallbladder polyps, which are fixed to the wall. Ultrasound has a sensitivity of approximately 95% for gallstones greater than 2 mm. Key associated findings include gallbladder wall thickening and pericholecystic fluid when cholecystitis is present.