While in health the relationship of alveolar ventilation to perfusion (VA/Q) is balanced and gives a ratio close to one, this balance can be disrupted in disease due to a decrease in either the ventilation or the perfusion of the lungs. Any such condition can cause an abnormal VA/Q ratio, either low or high, both of which will clinically result in hypoxaemia. In neonatal intensive care, where prematurity causing acute and chronic lung morbidity is the most commonly encountered type of respiratory disease, ventilation is typically more severely affected compared to perfusion, giving rise to hypoxaemia with a low VA/Q. Perfusion abnormalities can also persist over time and are associated with abnormally high VA/Q relationships, such as in congenital diaphragmatic hernia. The VA/Q ratio is conventionally calculated by nuclear medicine scans but can also be estimated by non-invasive bedside methods.