Flavonoids are phenolic compounds that represent substantial constituents of the non-energetic part of the human diet. They are naturally found in vegetables, berries, fruits, wine and beer. There are more than 5000 different flavonoids. The average intake depend on the country but the average intake is approximately 23 mg/day; quercetin is predominant at 16 mg/day. Flavonoids were considered initially to be substances without any benefit for humans. Later, it has been reported that they exert multiple biological effects due to their antioxidant and free radical-scavenging abilities. Although results from different studies have demonstrated that flavonoids can act as pro-oxidant at very high doses, most investigations have reported anti-inflammatory, antiviral, or anti-allergic effects and a protective role in heart diseases, cancer and different pathologies.