This chapter defines the elements of the brain-gut axis (BGA), how they interact, and how dysfunction of this axis may contribute to functional gastrointestinal disorders (FGIDs). It reviews the peripheral and central components of the BGA. Patients with FGIDs tend to focus on possible causes of their symptoms at the level of the gut, although many recognize a relationship between psychological stress and the onset or exacerbation of their symptoms. The enteric nervous system (ENS) consists of two nerve plexuses between layers of the gut wall that communicate with the central nervous system (CNS) through multiple neural and endocrine pathways. 'Visceral hypersensitivity' refers to painful perceptions arising from gastrointestinal stimuli at a lower threshold than that observed in health, and is commonly observed in patients with FGIDs. Peripheral sensitization tends to be transient, so that further changes would be required to perpetuate the hypersensitivity in chronic illness, such as an increase in mast cells and inflammatory mediators.