This chapter presents information on the role of symbiotes in insect-plant interactions with emphasis on the endosymbiotes. Extracellular symbiotes between microorganisms and insects has developed both internally and externally in insects. The two most thoroughly studied groups of herbivorous insects which possess endosymbiotes are the Homoptera and Coleoptera. As in nonherbivorous insects, the presence of endosymbiotes in certain members of these herbivorous insects generally connotes a relatively restricted diet confined to one or two particular tissue-types of the host plant. Symbiotic microorganisms of beetles appear to be chief producers of an array of digestive enzymes which are capable of the hydrolytic breakdown of plant matrix polysaccharides. Most of the studies on the acquisition of digestive enzymes from saprophytic or mutualistic ectosymbiotic bacteria or fungi have been by Michael M. Martin and colleagues at the University of Michigan. The role of extracellular symbiotes associated with the alimentary tract of dipteran larvae and/or adults is only partially understood at present.