Abstract: Cynipidae are Hymenoptera which cause malformations in different vegetal organs, although some of them have lost the capacity to make the galls themselves being inquilines of other cynipid galls. A review of the Cynipidae species potentially harmful for the Mediterranean oak forests is made. The harmful Cynipini species worldwide are pointed out and why some species producing very similar galls do not damage Quercus is discussed. The most important morphological features and the biology these species are presented. The risks to the plants are discussed in terms of organs bearing the galls.