Flavonoids are a group of plant antioxidants that are widely distributed in plants from the root and stem to the leaves, flowers, and fruits. They are generally present as glycosides, conjugated to sugars, although their aglycone forms may also exist. Flavonoid glycosides are hydrolysed from sugar moieties in the intestine. After release from sugars, the flavonoid aglycones undergo conjugation with other molecules to facilitate the excretion and shorten the half-life of flavonoids. Conjugation also reduces the antioxidant and potential pro-oxidant effects of flavonoids. Flavonoids have low intestinal bioavailability and rapid urinary and biliary excretion. However, the bioavailability of flavonoids varies between different kinds of flavonoids. The best rate of absorption among flavonoids is seen in gallic acid and isoflavones, followed by catechins, flavanones, quercetin, proanthocyanidins, and anthocyanins. Quercetin metabolites are excreted very slowly, whereas anthocyanins and catechins are excreted very rapidly. Information on bioavailability and metabolism of flavonoids can help designing the best intervention strategies. In conclusion, flavonoids have low intestinal bioavailability and rapid urinary and biliary excretion. The bioavailability of them varies between different kinds of flavonoids. The most rapid excretion rates belong to anthocyanins and flavanols while the slowest rates are for flavonols.