Gastroscopy and enteroscopy can only see the most superficial layer of the digestive tract (the mucosal layer); some lesions have a smooth surface and originate from the submucosa (e.g., submucosa, muscularis propria, plasma layer); or some lesions are located outside the lumen of the stomach in close proximity to the gastric wall. Ultrasound endoscopy is a combination of ultrasound probe and endoscopic technology. When the endoscope is sent into the digestive tract, both the mucosal surface lesions can be directly observed through the endoscope, and real-time ultrasound scanning can be carried out in order to observe the ultrasound images of the various layers of the tissue structure of the wall of the digestive tube and its neighboring organs, which further improves the diagnostic level of endoscopy and ultrasound. Ultrasonic endoscopy is the most promising new technology of digestive endoscopy. 1.Indications (1)Malignant tumors of the digestive tract (e.g. esophageal cancer, gastric cancer, colon, rectum): TNM staging is performed, but the role of M staging is limited in order to assess the surgical resectability, prognosis and guide the choice of treatment options. (2) Submucosal tumors (e.g., smooth muscle tumors, etc.); to determine whether they are extra-tubular wall lesions, organ compression, or lesions of the tubular wall itself; to determine the exact origin, nature, and extent of the lesion; and to guide the choice of treatment options. (3) Pancreatic lesions: it is a better auxiliary diagnosis and differential diagnosis for the diagnosis of chronic pancreatitis, and can better reflect the changes in the parenchymal structure of the pancreas. (4) It can show some mediastinal lesions. Contraindications The contraindications of ultrasonic endoscopy of the digestive system are basically the same as those of ordinary gastroscopy. 3.Diagnostic value (1)Submucosal tumor: EUS is the preferred method to diagnose submucosal tumor. Through EUS, we can easily exclude the artifacts caused by extra-luminal compression, and for submucosal tumors, it can clearly show that the lesion originates from that layer of the structure of the wall of the digestive tract, and the size, shape, edges and echogenicity of the lesion. (2) Esophageal cancer: To determine the depth of infiltration of the lesion into the wall of the tube and whether there are enlarged lymph nodes around the lesion is the most important feature of EUS examination. (3) Pancreatic disease: traditional pancreatic examination methods include body ultrasound, CT, MRI and retrograde pancreatography (ERP). The application of pancreatic EUS must be contrasted with traditional pancreatic examination methods to complement their strengths and weaknesses in order to arrive at an optimal diagnosis. It is now recognized that EUS is the most sensitive method for detecting microscopic lesions in the pancreas, and can detect lesions as small as 2-3 mm. Fine-needle aspiration cytology under the guidance of EUS can make a qualitative diagnosis of microscopic lesions in the early stage of the pancreas. EUS can be used to stage pancreatic cancer for TNM and determine the likelihood of surgical resection. For pancreatic cysts, EUS not only has diagnostic value, but also can perform puncture internal drainage and other treatments. (4) Mediastinal lesions: the diagnosis of mediastinal lesions mainly relies on CT, MRI and other non-invasive examinations, but EUS has an important role in the localization and qualitative diagnosis of mediastinal lesions, especially in the middle and posterior mediastinum around the esophagus to show the lesions more accurately. The most clinically valuable is fine-needle aspiration cytology under the guidance of EUS, which is important for the diagnosis of a variety of diseases such as metastatic lymph nodes of malignant tumors of unknown origin in the mediastinum, lymphoma, and nodal disease.