Abstract : Flavonoids represent a major class of plant polyphenols that are distributed ubiquitously in the human diet. Interest in the possible health-promoting effects of flavonoids has increased due to their various bioactive properties, as demonstrated in several in vitro and in vivo studies. Knowledge about flavonoid absorption and bioavailability is essential in order to fully understand their potential health benefits. This thesis was concerned with the uptake and absorption of dietary flavonoids in humans, with an emphasis on the effect of the chemical form of the flavonoid (as a product of processing) and the effect of food matrix. The first study investigated the flavonol content in different varieties of commercially-grown tomatoes and widely consumed tomato-based food products. It was shown that the levels of rutin varied considerably between different cultivars and food-based products, with the highest levels found in cherry tomatoes. Furthermore, cellulase hydrolysis of rutin was shown to be a viable alternative for the release of quercetin aglycone. The second study was aimed to investigate the absorption and bioavailability of dietary quercetin glycosides and its aglycone from a food-based system (non-supplement) in humans by implementing urinary time-course analysis. Ingested quercetin glycosides resulted in an observed earlier appearance of quercetin conjugates in urine then when quercetin aglycone was ingested (1 h compared to 2 h). Significantly more quercetin was absorbed than the aglycone when present in a similar onion-based food matrix system (1.8% compared to 1.3%). Finally, a second human study was carried out to investigate the effect of dietary fat, 8.3% (w/w), on the bioavailability of quercetin present in onion-based foods as either quercetin glycosides or its aglycone. Absorption of quercetin in its glycosidic form was shown to be significantly higher than the aglycone form when co-ingested with fat (2.8% compared to 1.7%). The ingestion of quercetin with fat, compared with the study without fat, enhanced the bioavailability of quercetin glycosides and its aglycone by about 50 and 23%, respectively, which was significant for the quercetin glycoside meal only.