Endoscopic ultrasound (EUS) has developed significantly over the last two decades and has had a considerable impact on the imaging and staging of mass lesions within or in close proximity to the gastrointestinal (GI) tract. In conjunction with conventional imaging such as helical computed tomography and magnetic resonance imaging, the indications for EUS include (1) differentiating between benign and malignant lesions of the mediastinum and upper GI tract, (2) staging malignant tumors of the lung, esophagus, stomach, and pancreas prior to surgery or oncological treatment, (3) excluding common bile duct stones before laparoscopic cholecystectomy, thereby avoiding the need for endoscopic retrograde cholangiopancreatography (ERCP) in some patients, and (4) assessing suspected lesions that are either equivocal or not seen on conventional imaging. In recent years, EUS has charted a course similar to that taken by ERCP, evolving from a purely diagnostic modality to one that is interventional and therapeutic. These indications include (5) obtaining a tissue diagnosis by EUS-guided fine-needle aspiration or trucut-type needle biopsy and (6) providing therapy such as coeliac plexus neurolysis and pancreatic pseudocyst drainage--in many cases, more accurately and safely than conventional techniques. Emerging investigational techniques include EUS-guided enteric anastomosis formation and fine-needle injection therapy for malignant disease.