Endoscopic resection has been accepted as both minimally invasive and curative treatment modality for early gastric cancer (EGC). The widely accepted indication of endoscopic resection for EGC is small sized, differentiated mucosal cancer in which the risk of lymph node metastasis is negligible. Tumor size can be measured by conventional endoscopy, and chromoendoscopy, magnifying endoscopy, narrow band imaging, autofluorescence imaging can also be helpful for accurate estimation of tumor size. Pretreatment tumor histology can be assessed with endoscopic biopsy, and also be measured by confocal endomicroscopy (so called "virtual biopsy"). Although endoscopic ultrasonography may be helpful for the assessment of tumor depth in EGC, the accurate assessment of tumor depth can be performed by the typical findings in the conventional endoscopy, by which treatment modality can be decided according to the depth of tumor invasion.