Real-time histological diagnosis is of paramount importance in the real-time decision of treatment plan for colorectal polyps. Neoplastic polyps should be endoscopically resected whereas diminutive hyperplastic polyps < 5 mm in the rectosigmoid colon can be left in situ because of absence of malignant potential. Characterization of colorectal polyps is performed by a variety of optical diagnosis methods including white light endoscopy, image enhanced endoscopy, and dye chromoscopy, etc. This lecture will cover these methods and discuss several representative cases to practically understand the characterization steps for real-time histological diagnosis.