Flavonoids are polyphenolic compounds that are mainly derived from fruits and vegetables and constitute an essential part of plant-derived beverages such as green tea, wine and cocoa-based products. They have been shown to possess anticancer effects via different mechanisms such as carcinogen inactivation, antiproliferation, cell cycle arrest, induction of apoptosis and differentiation, inhibition of angiogenesis, anti-oxidation and reversal of multidrug resistance or a combination of any two or more of these mechanisms. The present review summarizes the chemistry, biosynthesis and anticancer evaluation of flavonoids in both animal and human studies. A special emphasis has been placed on the flavonoids that are being screened in different phases of clinical trials for chemoprotective action against various cancers.