Insects engage in intimate associations with microbial symbionts that colonize their digestive systems or internal cells and tissues. The stability and near ubiquity of many of these “symbioses” implies their importance, a prediction supported through experimentation. With the advancing power of experimental methodologies and the growing accessibility of genomic techniques, insect science has reached a powerful new stage enabling the study of previously recalcitrant symbioses, including several with medical and agricultural significance. In this volume we publish a collection of chapters focused on the physiology of insect-microbe symbioses, emphasizing their mechanistic underpinnings, and the ecological and evolutionary causes and consequences of these interactions. Resident microbes modulate insect digestion, nutrition, detoxification, reproduction, interspecies signaling, and host-parasite interactions, and these chapters synthesize impactful, state-of-the art research on insect-microbe symbioses. Through discussions of the mechanisms that both stabilize and regulate these symbioses, these chapters yield further insight into the physiological integration between many insects and their influential microbial partners.