Insect-induced galls in plants represent highly regulated growth manifestations in response to insect feeding or egg laying. Physiological changes and signal-activated reactions in plants result in the expression of galls that serve the nutritional and shelter needs of the inducing insect. Variations in differentiation patterns occur in galls induced by insects of different taxonomic groups. Fascinating diversity of gall-inducing insects and their galls has attracted attention of many researchers. In some instances plants suffer extensive damage affecting their performance and productivity and even experience death. Keeping these aspects in mind the present volume highlights the current situation of gall inducing arthropods on forest trees, a reassessment in terms of their status, associated natural enemies, spread to other related and unrelated plants particularly forest tree species. The chapters provide insights into the interactions of some of the gall inducing insect species with trees.