Endoscopic ultrasound (EUS) is an evolving technique used by gastroenterologists to examine lesions that are located either within or adjacent to the walls of the upper gastrointestinal (GI) tract; this topic is relatively unknown to most radiologists. Proper use of this modality is benefited by a cooperative effort between gastroenterologists and radiologists specializing in ultrasound and cross-sectional imaging. This article informs radiologists of the applications of this procedure. Most patients are examined with EUS after a biopsy of a mucosal tumor has been performed. A smaller number are performed to evaluate submucosal masses or when pancreatic disease is suspected but not diagnosed. The examinations can be performed either with dedicated flexible echoendoscopes or with catheter-based probes passed through a conventional endoscope. The exact location of abnormalities associated with the upper GI tract can be observed. Known anatomic landmarks are sought. Abnormalities of structures outside the upper GI tract will occasionally be found during these examinations. The specific layers of the walls of the gut are examined, and the T and N-classification of upper GI tumors can be determined accurately. The performance of an EUS examination requires advanced skills, and in many medical centers, it is the imaging modality of choice to stage cancers, to evaluate submucosal masses, and to investigate both malignant and benign pancreaticobiliary disease. Endoscopic ultrasound is sensitive but not specific, and biopsy is necessary to establish a diagnosis. Therapeutic applications of EUS are evolving. Specialized applications with catheter-based probes are also being developed.