Quercetin is a plant flavonoid phytochemical exhibiting a broad spectrum of properties i.a. antioxidant, anti-inflammatory and immunomodulatory. However, the effect of quercetin is not clear. This compound at low concentrations can stimulate proliferation of human cells, so it can be a potential drug in the treatment of neurodegenerative diseases and in high concentrations, it induces apoptosis thereby eliminating the infected or abnormal cells and can serve as a potential anticancer drug with wide clinical application. Action of quercetin can be explained by its interference with cellular enzymes, receptors, transporters and signalling system. Due to its widespread occurrence in the plant world, it is an integral component of the human diet. The dietary quercetin occurs most often in the form of β-glycosides connected mostly with rutinose, rhamnose and glucose. Depending on the nutritional habits, the daily intake of flavonoids, including quercetin, ranges from 3 to 70 mg. Epidemiological studies confirm an inverse correlation between the consumption of flavonoids and the incidence of lifestyle diseases and tumor formation. Published data indicate that consumption of foods rich in flavonoids reduces the risk of coronary heart disease. Thus, flavonoids - including quercetin - seem to be an interesting pro-health agent.