Insects are among the most successful animals on Earth both with regard to their biomass and biodiversity. It is estimated that up to 20% of all insects are obligately associated with symbiotic microorganisms, and it is likely that their capacity to engage microbial companions has greatly contributed to their evolutionary success. The main focus of this review lies on obligately intracellular bacteria residing in specialized cells, the bacteriocytes, provided by the host. In the past few years the focus in research on these bacteria has been on their biological role for the host and the consequences on the genome and metabolic capacities shaped by a long-lasting obligate association confined to the interior of a eukaryotic host cell. Here, we compare those endosymbiont-host interactions where the genome of the bacterium is sequenced.