Quercetin (2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4-H-1-benzopyran-4-one) is a flavonoid present in the plant kingdom in the glycoside form. Main group of quercetin derivates are quercetin O-glycosides, and the main place where glycosylation occurs is C3. The aim of the thesis was to present in details main information regarding quercetin structure and properties, biochemistry and synthesis, bioavailability and metabolism. Scientific studies on quercetin bioactivity were analysed and an insight on the importance and impact of a diet rich in quercetin on health was given, interactions of quercetin with medication and its potential toxic effect were also discussed. Quercetin is considered as a powerful antioxidant with a pronounced capability of free radical elimination and chelation of metals. Aside from antioxidant, quercetin also possesses antiviral, anticancer, anti-inflammatory and other pharmacologically significant properties that were also discussed in the thesis. Results of scientific studies that deal with opposed activity of quercetin were also analysed, and they showed that, depending on the dosage, quercetin is a potential remedy for the prevention and treatment of various illnesses, such as neurodegenerative and cardiovascular and many other.