The microbiota-gut-brain axis is emerging as a research area of increasing interest for those investigating the biological and physiological basis of brain development and behaviour during early life, adolescence & ageing. The routes of communication between the gut and brain include the vagus nerve, the immune system, tryptophan metabolism, via the enteric nervous system or by way of microbial metabolites such as short chain fatty acids. Studies in animal models have shown that the development of an appropriate stress response is dependent on the microbiota. Developmentally, a variety of factors can impact the microbiota in early life including mode of birth delivery, antibiotic exposure, mode of nutritional provision, infection, stress as well as host genetics. Recently, the gut microbiota has been implicated in regulating the stress response, and social behaviour. Moreover, fundamental brain processes from adult hippocampal neurogenesis to myelination to microglia activation have been shown to be regulated by the microbiome. Further studies will focus on understanding the mechanisms underlying such brain effects and how they can be exploited by microbiota-targeted interventions including ‘psychobiotics’ and diet