Symbioses occur in all higher organisms and have evolved to play an important role in host biology. Researchers have been studying the effects that microbial symbionts have on host biology for decades but have only recently begun to examine how they influence the brain and behaviour. This review aims to provide several examples of different symbionts that have demonstrated the ability to manipulate the behaviour of their hosts and described the current evidence for the molecular mechanisms used by the symbiont to alter the host's nervous system and modify behaviour to illustrate the common points of interaction between symbiont and host.