Endoscopic ultrasound (EUS) is a superior modality for local staging of gastrointestinal cancer. In interventional endosonography linear array echoendoscopes permit real-time EUS-guided puncture of target lesions for cytologic evaluation of such lesions. This article describes the basic principles of EUS, established indications pertaining to gastrointestinal cancer and other malignancies, and emerging indications for this minimally invasive technology.