Phenols may contribute to the chemopreventive effects exerted by olive oil against cancer. Hydroxytyrosol and other olive oil phenolic compounds demonstrated both anti-initiation and antipromotion/progression properties. The anti-initiation effect consists of the prevention of oxidative DNA damage and is mediated by the antioxidant capacity of phenols exerted at low concentrations. The antipromotion/progression effect depends on the capability of phenols to inhibit the proliferation and to induce apoptosis and differentiation in tumor cells at high concentrations. The mechanisms involved in the antipromotion/progression characteristics of phenols include both alteration of the gene expression and modulation of the phosphorylation state of important targets of the signal transduction pathways.A pro-oxidant activity of olive oil phenols at high concentrations has been hypothesized.