Insects are by far the most diverse and abundant animal group with respect to the number of species globally, in ecological habitats and in biomass. The ecological and evolutionary success of insects depends in part on their countless relationships with beneficial microorganisms, which are known to influence all aspects of their physiology, ecology, and evolution. These symbiotic associations are known to: (a) enhance nutrient-poor diets, (b) aid digestion of recalcitrant food components, (c) protect from predators, parasites, and pathogens, (d) contribute to inter- and intraspecific communication, (e) affect efficiency as disease vectors and (f) govern mating and reproductive systems. Characterization, exploitation, and management of the insect-bacterial symbiotic associations can contribute significantly to the control of agricultural pests and disease vectors. Insects that depend exclusively on nutritionally restricted diets such as plant sap, vertebrate blood, and woody material, commonly possess obligate mutualistic endosymbionts involved in the provision of essential nutrients or in the degradation of food materials. These intracellular mutualists commonly have the following biological features: (a) they localize inside bacteriocytes, (b) are essential for fitness, (c) are maternally transmitted, and (d) display strict host-symbiont co-evolutionary patterns. In addition to obligate endosymbionts, many insects harbor bacteria that are not essential for their survival or fecundity and are typically maintained with a patchy distribution in host populations. Such symbionts can induce reproductive phenotypes in insect hosts, including male-killing, feminization, parthenogenesis or cytoplasmic incompatibility. Because these bacteria manipulate their host’s reproductive biology, they also likely accelerate host processes. As for essentially all animals, microbial communities are particularly prominent in the digestive tract, where they may be key mediators of the varied lifestyles of insect hosts. The contribution of microorganisms, particularly gut microorganisms, to insect function is highly relevant from several perspectives, linking to applications in medicine, agriculture, and ecology. Gut-associated microorganisms can include protists, fungi, archaea, and bacteria, but it is generally accepted that bacterial species dominate the microbial community in the guts of most insects. Gut-associated bacteria can influence: (a) vectoring efficiency, (b) developmental time, (c) decomposition of plant biomass and carbon cycle, (d) nitrogen fixation and nitrogen cycle, (e) mating incompatibilities, and (f) detoxification of pesticides leading to the acquisition of insecticide resistance.