The branching network of airways in the lung poses a complex challenge to both physiologists and physicians. Measurements of lung function have been made for nearly 200 years. Davy (1800) and Hutchinson (1844) were amongst the first to recognise and measure lung volumes. Despite these early discoveries, progress in understanding pulmonary physiology was intermittent. Rohrer (1925) and Rahn (1946) established the concepts of pressure and volume relationships in the lung and Rohrer alluded to the possibility of volume dependent airways resistance, although he did not live long enough to make these measurements himself.1 ,2 In fact, it was not until the 1960s that the issue of airways resistance was resolved and the distinction drawn between the large and small airways.3 ,4Small airways are usually defined as those airways of less than 2 mm internal diameter and equate to generations 12–23 of Weibel’s model of lung architecture.3 Although the small airways are themselves tiny, the dichotomously branching pattern of the tracheobronchial tree results in increasingly large numbers of airways in peripheral generations. This results in very low resistance of each generation of small airways, so that in total they contribute only about 10% of the total airway resistance of the tracheobronchial tree.5 ,6 Thus, most of the small airways can be damaged or destroyed before symptoms occur and before any of the conventional tests of lung function, in particular measurements of airway resistance, show loss of function. …