Hereditary bacterial symbiosis is a common mechanism by which eukaryotic hosts can acquire traits beneficial for their fitness. Many insects have symbiotic associations with bacteria that trace back millions of years, whose function and evolution are well characterized. Insects can also possess more recently derived symbionts that are closely related to free-living bacteria, and often play a role in host defense. Serratia symbiotica is a recently derived symbiont that infects aphids and provides protection against heat stress, and possibly also plays a nutritional role. Many aspects of the biology of recent symbionts are less well studied, including the diversity of functional roles and evolution among hosts for single lineages of symbionts, the molecular mechanisms that contribute to defense, the early stages of symbiont genome evolution, and interactions with hosts. This dissertation focuses upon S. symbiotica to contribute research addressing each of these themes. Functional studies revealed that S. symbiotica lysis during heat-shock is correlated with protection of the nutritional symbiont Buchnera , and that S. symbiotica has a large effect upon aphid metabolite pools. Despite this large metabolic effect, S. symbiotica does not seem to dramatically influence expression of aphid genes, including those involved in immunity. Analysis of the evolution of S. symbiotica lineages in different aphid hosts revealed this symbiont is common in the aphid subfamily Lachninae, but did not support the obligate nutritional role hypothesized in the literature for this group. Finally, comparison of the S. symbiotica genome to close free-living relatives revealed a genome undergoing massive decay, and provided a rare opportunity to examine the evolution of a recently acquired symbiont.