The concept of dysplasia-carcinoma sequence has been established for the gastrointestinal tract. Dysplasia is defined as unequivocal neoplastic proliferation of epithelium without invasion and represents the precancerous lesions of the colon and rectum. The most common appearance of dysplasia is polypoid adenoma, however, flat adenomas are increasingly diagnosed. During the last 20 years, new pathological and biological methods including molecular genetics showed the stepwise evolution of colorectal carcinoma from normal mucosa to dysplasia of increasing grade and to invasive carcinoma. Of course, dysplasia-carcinoma sequence does not imply the development of carcinoma in every focus of dysplasia. In fact, the incidence of carcinomas from adenomas is 5-10%. A new molecular or genetic epidemiology promises an improved selection of high risk individuals.