The rapid growth in microbiome research, particularly during the last 15 years, has revealed the crucial contributions of microbial communities to numerous physiological functions in animals, including digestion, immunity and reproduction. The permanent coexistence of these various bionts forms the holobiont (namely, the host and its microbiota). This review describes the relationships between xylophagous insects and their microbiota in an attempt to understand the characteristics that have determined bacterial fidelity over generations and throughout evolutionary history. Symbiotic interactions have probably played a central role in the evolutionary success of these insects, allowing their adaptation to unexploited ecological niches that are nutritionally deficient and/or unbalanced. Moreover, insect symbionts have provided the enzymatic capabilities that enable the synthesis of nutrients (carbon and nitrogen sources for the host) from recalcitrant plant polymers (cellulose, hemicellulose, pectin or lignin).