Endoscopic ultrasound (EUS) is a minimally invasive tool used to diagnose and stage gastrointestinal and lung cancers. EUS can accurately determine how deeply a tumor penetrates the gut wall and helps determine whether cancer has spread to adjacent lymph nodes. Accurate staging is crucial in determining what type of therapy is appropriate. Early-stage tumors can often be treated with endoscopic therapy or surgery. Advanced-stage tumors usually require chemotherapy and radiation, followed by surgery. On occasions, detecting advanced disease with EUS may prevent unhelpful and invasive therapy. EUS can also be used to diagnose the cause of abdominal pain or abnormal weight loss, such as gallstones, bile duct stones, or chronic pancreatitis.