Flavonoids are polyphenolic compounds whose main dietary source are fruits and vegetables. Epidemiological evidence has suggested that dietary flavonoids may protect against heart disease but biological effects have not hitherto been demonstrated directly in humans and there was no consistent evidence about the absorption of flavonoids. The studies performed for this thesis aimed to test antioxidant properties of flavonoids using an in vitro system, ex vivo tests on human tissue (lymphocytes) and in a dietary intervention. The main conclusions of this thesis are: 1. There is a potent antioxidant action of dietary flavonoids demonstrated by the comet assay, of potential importance in protection against cardiovascular disease and cancer. 2. The antioxidant capacities of most major dietary flavonoids are greater than vitamin C. 3. Results from the comet assay and TEAC show reasonable agreement in ranking. 4. Antioxidant activities of free flavonoids are more than the conjugated flavonoids. 5. There was a strong positive correlation between the number of hydroxyl groups of flavonoids and the antioxidant activity. 6. Dietary flavonoids are absorbed and the fasting plasma concentration can be increased 12 fold by a simple and palatable food supplement. 7. Supplementation with onions, tomato ketchup and tea led to protection of lymphocytes against free radical damage (H2O2), a biological effect of potential medical importance possibly attributable to the absorption of dietary flavonoids. 8. Dietary flavonoids intake (and specifically quercetin) can be estimated with reasonable accuracy from 24 hour urinary flavonoid excretion or fasting plasma concentration. 9. The range of dietary flavonoid consumption in ten NIDDM patients was estimated at 20-80 mg/day from their normal diets. On the basis of results in this thesis, dietary difference within this range would influence tissue antioxidant status.