Flavones and isoflavones may play a prominent role in cancer prevention since these compounds are found in numerous plants that are associated with reduced cancer rates. This article reviews recent epidemiological and animal data on isoflavones and flavones and their role in cancer prevention. It covers aspects of the bioavailability of these dietary constituents and explores their mechanism of action. Human epidemiology data comes primarily from studies in which foods rich in isoflavones or flavones are associated with cancer rates. This approach has been particularly useful with isoflavones because of their abundance in specific foods, including soy foods. The bioavailability of flavones and isoflavones has been shown to be influenced by their chemical form in foods (generally glycoside conjugates), their hydrophobicity, susceptibility to degradation, the microbial flora of the consumer, and the food matrix. Some information is available on how these factors influence isoflavone bioavailability, but the information on flavones is more limited. Many mechanisms of action have been identified for isoflavone/flavone prevention of cancer, including estrogenic/antiestrogenic activity, antiproliferation, induction of cell-cycle arrest and apoptosis, prevention of oxidation, induction of detoxification enzymes, regulation of the host immune system, and changes in cellular signaling. It is expected that some combination of these mechanisms will be found to be responsible for cancer prevention by these compounds. Compelling data suggest that flavones and isoflavones contribute to cancer prevention; however, further investigations will be required to clarify the nature of the impact and interactions between these bioactive constituents and other dietary components.