Wolbachia pipientis is a maternally inherited obligate intracellular bacterium and a common endosymbiont of many terrestrial arthropods. Wolbachia induce a diverse range of reproductive changes in their hosts, including parthenogenesis, male killing, feminisation and cytoplasmic incompatibility (CI). Female hosts infected with Wolbachia gain fitness advantages from these manipulations and, because this bacterium is maternally inherited, these infections tend to spread. The capacity to spread makes Wolbachia an attractive organism for a range of biocontrol strategies. Recently, two strains of Wolbachia, wMel and wMelPop, have been transinfected into the mosquito Aedes aegypti Linnaeus (Diptera: Culicidae) as part of a strategy to control the spread of dengue fever. In its novel mosquito host, Wolbachia has been associated with pathogen interference against many pathogens – including dengue viruses – and cytoplasmic incompatibility, the reproductive manipulation that may facilitate the spread of this bacterium into host populations. However, Wolbachia is also associated with changes in mosquito fitness including reductions in egg viability and, in adults, reductions in bloodfeeding success, fecundity and lifespan. In other insect species, Wolbachia infections have been associated with changes in host immunity, locomotion, dispersal and foraging and mating behaviours. These Wolbachia-mediated changes potentially undermine the fitness advantages of pathogen interference and cytoplasmic incompatibility, and such changes could reduce or prevent the spread of Wolbachia into vector populations. In this series of studies, I investigated the influence of Wolbachia infections on the reproductive success, mating behaviour, host-seeking behaviour and gene expression of Ae. aegypti. In chapter 2, population mating experiments showed that Wolbachia infections in males did not affect male mating frequency or male fertility. However, the fecundity of females and egg viability were reduced by Wolbachia infections in female Ae. aegypti. In chapter 3, Y-tube olfactometer experiments showed that Wolbachia did not influence the host-seeking behaviours of Ae. aegypti in response to synthetic or natural host-odour cues. In chapter 4, I examined whether Wolbachia induces changes in expression of a major circadian-rhythm controlling gene and several other candidate genes that may explain wMelPop-associated neurodegenerative phenotypes. However, the experiments found the presence of infection did not alter circadian rhythm-associated gene period (per), nor the candidate genes swisscheese (sws), vacuolar penduncule (vap), Kynurenine hydroxylase (kh) and neurexin IV (nrxIV). Finally, in chapter 5, I attempted to examine the effects of Wolbachia on the midgut microbial communities of Ae. aegypti but failed to successfully complete this study due to technical difficulties. The results are discussed in the context of how they may relate to current and future field releases of Wolbachia-infected Ae. aegypti and are encouraging for the potential of Wolbachia to reduce the transmission of vector-borne diseases.