Quercetin (2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-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 this work is to present detailed information regarding quercetin structure, properties, biochemistry, bioavailability and metabolism. Scientific studies on quercetin bioactivity are analyzed 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 are 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. Results of scientific studies that deal with opposed activity of quercetin are analyzed, 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 others.