In order to ascertain the role of dietary flavonoids as antioxidants in vivo it is necessary to understand the chemical nature of the absorbed forms in the circulation in vivo and how the multiplicity of research findings in vitro reflect the bioactivity of flavonoids in vivo. Only when we gain adequate information on the circulating forms can we begin to understand the targeting to the tissues, whether flavonoids cross the blood-brain barrier, for example, and in what forms. Flavonoids are powerful antioxidants in vitro, but their overall function in vivo has yet to be clarified, whether antioxidant, anti-inflammatory, enzyme inhibitor, enzyme inducer, inhibitor of cell division, or some other role. It should also be emphasised that the reducing properties of flavonoids might contribute to redox regulation in cells, independently of their antioxidant properties, and thus might protect against cell ageing, for example, by working together with the intracellular reductant network. To gain understanding of these issues the factors influencing the absorption of flavonoids in the gastrointestinal tract needs to be established, namely the questions of: de-glycosylation before absorption, conjugation in the small intestine through glucuronidation, sulphation or methylation etc, metabolism and degradation in the colon to smaller phenolic molecules. The forms in which they circulate in vivo will influence their polarity and, thus, their localization and bioactivities in vivo. Finally if antioxidant activities are important, the elucidation of how such properties in vitro relate to the potential for conjugates and metabolites in vivo to act as antioxidants is required. The absorbed flavonoid components might function in the aqueous phase (like vitamin C) or in the lipophilic milieu (as vitamin E) in vivo. This will depend on their polarity properties on uptake, how they are metabolised on absorption, and their resulting structural forms in the circulation.