The incidence of and mortality from colorectal cancer are increasing in Japan, as is its proportion among all malignant neoplasms. Thus, primary prevention of this cancer is crucial. Colorectal cancer is caused by interactions between host and environmental factors, with accumulation of gene alterations, such as activation of oncogenes and inactivation of suppressor genes, and generally involves an adenoma-carcinoma sequence. Carcinogenesis progresses with multi-factor, multi-hit and multi-stage mechanisms. According to the report by WCRF/AICR, convincing preventive factors include eating vegetables (not fruit) and physical activity (colon only), while probable risk factors are red meat and alcohol. Possible preventive factors include dietary fiber, starch and carotenoids, whereas possible risk factors include high body mass, fat and heavily cooked meat. Such preventive and risk factors for colorectal cancer are discussed in this review.