This book is a collection of contributions from leading scientists in the field of intracellular symbiosis research outlining the present status of endocytobiosis studies on the most important groups of insects. Emphasis is on the results of ex-perimental symbiosis research on the morphology, physiology, and genetics of selected insect groups. Insights provided by the new interdisciplinary research field of endocytobiology is considered in interpreting experimental results of symbiology. Various insect endocytobionts are discussed with regard to their evolutionary development, reflecting the current status of evolution research. Special attention is given to the phylogenetic and systematic classification of endocytobionts. Also discussed in comparison with the similar system of ticks are all endosymbioses found in roaches, termites, ants, cicadas, coccids, aphids, flies, and beetles.