Flavonoids are polyphenolic compounds that are found in fruits, vegetables, tea and wine. Because numerous studies have shown that flavonoids possess antibacterial, antiinflammatory, antiallergic, antimutagenic, antiviral and anti-cancer properties they represent an important part of the human diet in the fight against various diseases. This paper describes some of these properties, with the accent on the antitumor effects of flavonoids and their mechanisms of action. Cancer is the second leading killer in the world after cardiovascular disease and is a major health problem in both developed countries and developing countries. Fruits and vegetables as the main source of flavonoids in the human diet help prevent cancer. It should be noted that flavonoids as natural compounds, unlike other therapeutic agents show minimal side effects, they are easily absorbed and they are not aggressive to the human body like chemotherapy. Flavonoids can modulate many biological processes in the development of cancer, such as apoptosis, cell proliferation and cell differentiation. For these reasons during the past 2–3 decades flavonoids occupy more and more attention of scientists in the fight against cancer. However more studies are needed, especially in vivo studies to determine the mechanisms and interaction of the many effects of flavonoids to prevent the development and to fight against cancer and other diseases.