An important question being raised by nutritionists today is whether available scientific data support an important role for polyphenols in the prevention of pathologic conditions that represent an important public health burden, such as cardiovascular diseases, cancers, and osteoporosis. More broadly, when can we consider scientific knowledge sufficient to allow specific public health implications and recommendations? The history of the relationship between beta-carotene and cancer illustrates the complexity of the research process leading to the demonstration of a causal relationship between nutritional factors and the prevention of disease. The beta-carotene story, which has developed in the past 30 y, is particularly significant and illustrative because of apparent controversies that are far from resolved. This is an extremely interesting example from which many lessons can be learned. For beta-carotene, we need to collect sufficient information from experimental, clinical, and epidemiologic research before we support any specific public health recommendations. The same principles must be applied to recommendations regarding polyphenols (in particular, which polyphenols, at which doses, to achieve which benefits for which populations). If these questions are not answered, then we run the risk of needing to renounce recommendations regarding polyphenols in the future, damaging the credibility of nutritional recommendations for public health.