This publication explores many facets of the ever intriguing and enigmatic relationships between plants and their gall-forming herbivores. The research reported herein ranges from studies on classical biology and systematics of galling to molecular phylogeny, population genetics, and ecological and evolutionary theory. Human kind has much to learn and gain from understanding the fine details of how plants and their gallers interact. This publication is the result of an international symposium that was held August, 1997 in Hungary. It was organized under the guidelines of the International Union of Forestry Research Organizations by the Hungarian Forest Research Institute, Department of Forest Protection, and the North Central Research Station of the US Forest Service.