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Ultrasound is a pivotal study for evaluation of the biliary tree. In particular, the size of the extrahepatic bile duct is a critical measurement and has been a contentious issue since the early days of diagnostic ultrasound. This article reviews the history and ongoing issues regarding sonography of the normal-size duct and a variety of factors that may affect its size, including age, prior surgery, congenital abnormalities, anatomical variations, and medications. Other related sonographic issues are discussed including abnormal nondilated ducts and abnormal intraluminal contents such as sludge or air that make evaluation of the duct more difficult, particularly in patients with primary sclerosing cholangitis and prior liver transplantation. Ultimately, the luminal size of the extrahepatic duct should be considered as a single part of the entire assessment of the biliary tree that must also include the intrahepatic and pancreatic ducts, the pattern of dilatation (variable vs progressively dilated to a single point of obstruction), any wall thickening, intraluminal sludge, calculi or mass, and extraluminal compression. Clinical symptoms and abnormal laboratory values should prompt further evaluation despite a normal appearance of the bile duct, whereas pursuit of an isolated finding of an enlarged duct without supporting clinical data may not be warranted.