Although less commonly used in clinical practice than spirometry, lung volume measurements are helpful in assessing progress and response to therapy in situations where lung volume is likely to be increased, as in asthma or cystic fibrosis, or reduced secondary to pulmonary hypoplasia, pulmonary fibrosis, or musculoskeletal abnormalities. Their contribution to research has been considerable. The techniques most commonly used are those of inert gas dilution on rebreathing, applicable to children of all ages but tending to under-record in the presence of gas trapping. The gold standard is body plethysmography. This is relatively expensive and requires more technical skill, but will measure all the gas within the chest. Computer programmes have been developed to obtain a reasonable estimate of lung volume from chest X-rays alone. More accurate results are obtained by echo-planar imaging but this is only available in highly specialized units. Information on fetal lung volumes can be obtained using digitized ultrasound images and position sensors attached to the ultrasound probe.