Endoscopic sonography (EUS) can depict with excellent accuracy the normal anatomy and abnormalities of the pancreaticobiliary region. In the case of biliary obstruction, EUS has the unique ability to identify the cause of obstruction, to assess the regional spread in neoplastic obstruction, and to evaluate parenchymal, vascular, and ductal abnormalities. For these tasks EUS is superior to conventional sonography, CT, and also, for some indications, to MR cholangiography. EUS is very accurate in differentiating stones from other benign or malignant causes of biliary obstruction.EUS assesses the presence of hyperechoic stones within bile ducts. EUS defines the number, size, and location of stones and estimates the diameter of the common bile duct. The results of EUS are independent of stone size or dilatation of the common bile duct. In diagnosis of common bile duct stones EUS is as accurate as retrograde cholangiography, but it is safer, and has a lower failure rate. Therefore, EUS has been proposed as an effective noninvasive evaluation of patients at intermediate and high risk for choledocholithiasis. On the other hand EUS, due to its limited penetration, has an excellent diagnostic accuracy for middle-distal bile duct cancer, whereas it is much lower for proximal cancer: the accuracy for vascular invasion has been shown to be 91% in middle-distal cancer and 57% in proximal cancer. If biliary obstruction is due to a pancreatic head or ampullary tumor EUS can detect the tumor and define its size, depth of tumor infiltration, involvement of major vessels, lymph node involvement, presence of local metastases, vascular invasion.