Endoscopic ultrasonography uses high-frequency ultrasound to visualize the gut wall and the surrounding structures of the mediastinum, the abdomen and the pelvis. Echoendoscopes are available in two different designs. A radial scanning echoendoscope produces a 360 degree real-time view perpendicular to the shaft of the echoendoscope. A linear-array instrument produces a 100 degrees real-time view parallel to the shaft of the echoendoscope, permitting direct ultrasonographic guidance of fine needles exiting the biopsy channel. Endoscopic ultrasonography has been established as the preferred diagnostic tool for the evaluation of submucosal masses of the upper gastrointestinal tract and the rectosigmoid, for differentiating benign from pathologic thickened gastric folds and for locating pancreatic endocrine tumors. The widest application of endoscopic ultrasonography is in the diagnosis and staging of esophageal, gastric, rectal and pancreaticobiliary neoplasms. Endosonography is the most accurate modality available for determining the T and N stages of these tumors. The recent development of endoscopic ultrasound-guided fine-needle aspiration provides physicians with the ability to cytologically diagnose lesions visualized endosonographically and to confirm cancer staging with tissue.