The therapeutic approach to cancer is generally limited to the advanced phases of disease. The preventive strategies aim at eliminating or reducing the exposition to known carcinogenes. We act with pharmacological and/or with an accurate dietary education to induce cellular differentiation phenomena, cytostasis and apoptosis. Chemoprevention acts both on the inductive phase (metabolic activation, DNA adducts), as well as on the promotion/proliferation of the long pre-clinical period of latency (antioxidants, anti-inflammatory, retinoids, carotenoids, vitamins and micronutrients, hormones and hormonal inhibitors, polyamine inhibitors, ditholetions, isothiocyanates, telomerase inhibitors, etc). Unanimous agreement has been reached on the preventive role of retinoids in head and neck tumors and of the cervical uterus, of hormonal inhibitors in breast and prostate cancer, and of anti-inflammatory in colorectal cancer. New and more accurate parameters for the evaluation of results and individual applications of chemopreventive strategies are linked to the biological research of high-risk subjects (genetic damage) or increased individual susceptibility. Caution, instead, should be applied in the clinical trial planning. An increased risk in developing and dying of lung tumor in smokers has been shown for the use vitamin A. Many clinical studies have been started in order to establish an efficient chemoprevention in oncology, and with the early diagnostic programs, and the evaluation of genotypic and phenotypic alterations, encouraging results will be reached for the next millennium.