Quercusspecies (Oak) are deciduous or evergreen trees or shrubs. They are important members of most forests all over the world. The genus has received great attention since ancient times due to its medicinal, ecological and economical value. Preparations ofQuercus(oak) species are used as hemostatic, antidiarrheic, astringent, or antiseptic agents, as well as for the treatment of burns, and inflammation of oral and anal mucosa. This review provides an overview of the polyphenolic constituents isolated and identified fromQuercusspecies. In vitro and in vivo pharmacological studies are reviewed and discussed, focusing on anti-viral and anti-microbial, anti-diabetic, anti-inflammatory, hepatoprotective and anti-oxidative effects of extracts from these plant species. Most of the pharmacological effects can be explained by the presence of high amounts of tannins or flavonoids, and to a lesser extent by other secondary metabolites, present in all plant parts. However, future efforts should concentrate less on in vitro, more on in vivo (precilinical studies) and more clinical trials in order to confirm traditional use in the light of a rational phytotherapy.