Oak cynipids initiate the growth of highly complex galls on plants in the genus Quercus and related genera in the family Fagaceae. These galls support closed communities consisting of high diversities of gall inducers, inquilines and natural enemies. Much research has focussed on oak cynipid communities and they offer significant potential as model. tritrophic systems. Nevertheless, key questions regarding their evolution and ecology remain unanswered. (1) Communities associated with different gall wasps vary in composition and structure, but little is known about the processes generating such variation. In particular, the role that phylogeography of component species plays remains unclear. (2) Gall traits (such as structure, location, and phenology) may play important roles in community diversity, but there is little empirical evidence of this. (3) The role of competition in oak cynipid communities is poorly understood. Many oak cynipids share natural enemies, and indirect apparent competition for the avoidance of shared natural enemies may occur between gall wasp species. In addition, direct competition for limiting resources may occur between gall inducers and between their parasitoids. We summarise current understanding of oak cynipid communities, discuss the issues raised above and outline key questions for further research. (c) 2005 Gesellschaft fur Okologie. Published by Elsevier Gmbh. All rights reserved.