Oak galls are not only extraordinarily complex from a histological and morphological point of view, diverse and abundant in oak and holm oak forest ecosystems, but they also play an important role in preserving the biodiversity of ecosystems. In this study, we review the histological organization, physiological changes of plant tissues and their functionality during gall formation. There is also a quick review of the diversity of galls and the biology of the wasps that form these galls. These galls are hot spots of biodiversity in which the inducers interact with the inquilines, and numerous parasitoids from different families of microhymenoptera. Under these attacks, the gall wasps have developed adaptive complex defense mechanisms involving gall structures that are also discussed. The galls, when abandoned, are used by other arthropods to protect themselves from adverse atmospheric conditions or during molting periods. Finally, these galls are used as a food resource by xylophages (gall tissues) such as birds and micromammals (in search of larvae).