Duodenal tumor is relatively rare among the gastrointestinal tumors. And majority of duodenal lesions are small benign lesions, however, there are some cases of aggressive duodenal tumors. By analyzing large number of resected duodenal tumors, we found that WOS (white opaque substance) negative lesions or large lesions more than 13 mm were independent risk factors of having higher malignant potential which is categorized as Vienna classification Category 4 and 5. Therefore, we should select appropriate treatment option according to the morphology of the target lesion after carefully checking the target lesions using NBI magnification. When the lesion is WOS positive small lesion less than 12mm, we can remove it by safe and easy techniques such as cold forceps polypectomy or cold snare polypectomy. However, if the lesion exceeds 13mm or WOS negative, we should remove it completely by more reliable techniques. EMR and UEMR are good techniques for the lesions less than 20mm. And ESD is the most reliable technique for the lesions more than 21mm to achieve margin free en-bloc resection. Unfortunately, complication rate of ESD is extremely high in the duodenum due to poor maneuverability of the endoscope, thin duodenal wall and presence of digestive enzymes. I developed the water pressure method in order to overcome technical difficulties of ESD and also developed the string clip suturing method to avoid serious delayed complications. Those techniques are quite effective to achieve good clinical outcomes, but duodenal ESD should be done by selected expert since it is still technically very much challenging.