Endosonography consists of exploring the gastrointestinal wall and surrounding tissue with the aid of an ultrasound transducer introduced into the gastrointestinal lumen. High frequency ultrasound (more than 7 MHz) is used and provides high definition imaging. Its principal role is in oncology, and more particularly in the assessment of tumor extension. Regardless of whether it is the rectum, esophagus or stomach being considered, it is possible to predict the depth of tumor invasion in the wall in more than 80% of cases. The determination of invasion of adjacent organs is still somewhat imprecise. Endosonography provides better analysis of lymph nodes than CT scan (70 to 75% precision) and improvements will occur with better description of the criteria of malignancy. It appears interesting for the evaluation of pancreatic and biliary tumors and for follow-up of patients already treated for gastrointestinal cancer. Among its other indications, some have been well established (gastrointestinal submucosal tumors, giant gastropathy) and others require confirmation (villous tumors of the rectum, gastric ulcer).