Polyphenolic compounds are common in the diet and have been suggested to have a number of beneficial health effects including prevention of cancer, cardiovascular disease, diabetes, and others. For some dietary polyphenols, certain benficial effects are suggested by epidemiological studies, some are supported by studies in animal models, and still others are extrapolated from studies in vitro. Because of the relatively poor bioavailability of many of these compounds, the molecular basis of these beneficial effects is not clear. In the present review, we discuss the potential health benefits of dietary polyphenols from the point of view of bioavailability. Tea catechins, curcumin, and proanthocyanidins are used as examples to illustrate some of the problems that need to be resolved. Further research on both the biological activity and bioavailability of dietary polyphenols is needed to properly assess their usefulness for the prevention and treatment of disease.