Spirometry is a useful clinical tool for the assessment and monitoring of obstructive lung disease but provides no information on the underlying mechanisms causing obstruction. The role of an increase in airflow resistance can be assessed by body plethysmography, but this technique requires elaborate equipment, a skilled operator and considerable subject co-operation. A much simpler technique, impulse oscillometry (IOS) provides a more extensive assessment of pulmonary mechanics. With IOS small volume oscillations are applied at the mouth during ordinary tidal breathing; the instantaneous pressure/flow signal produced by the imposed oscillations (impedance ,Zrs) is analyzed into its in-phase (resistance, Rrs) and out-of-phase (reactance, Xrs) components. Because Rrs is measured over a range of frequencies, IOS has the potential to distinguish between large and small airway resistance. Changes in Xrs have been less studied, but there are suggestions that these also may distinguish between different types of airflow obstruction.