This chapter focuses on the actions of probiotics, live microorganisms that, when administered in adequate amounts to humans and animals, confer a health benefit. The potential molecular mechanisms, particularly those that support a role for probiotics in the management of psychopathology, are reviewed and recommendations made for future research in this area. The chapter explores how the gut microbiome contributes generally to health and well-being, and then focuses on the pathways connecting the microbiome to central nervous system function: the microbiome-gut-brain axis. In health, the human gastrointestinal tract is host to around 10 bacteria, known as the gut microbiota and representing a bacterial genome, or microbiome, that outnumbers the number of human genes by two orders of magnitude. It is well established that the gut microbiota plays an important role within the gastrointestinal tract. Reduced local tolerance due to changes in the gut microbiota, for example, is likely to contribute to inflammatory bowel disease.