Background: Pulmonary function testing may assist in the diagnosis and follow-up of respiratory diseases. The most commonly used method to date is spirometry because it has well-established protocols for its execution and interpretation. Spirometry can be performed in children younger than six years of age, but achieving a reproducible and reliable measure in these children is a challenge since we can observe reproducible respiratory maneuvers by most children only after the age of five years. Impulse oscillometry (IOS) has been introduced as an alternative technique to assess lung function with particular application to asthma. IOS is noninvasive, easily performed during tidal breathing and requires only minimal patient cooperation. IOS being effort- independent makes it feasible even in young children. It also obviates the problems with interpreting forced mid expiratory flow rates (FEF25-75) which are highly volume dependent, as for example in patients who perform an incomplete expiratory maneuver from total lung capacity to residual volume. IOS is a useful tool for the assessment of lung function in pre-school children mainly because it is effort independent, it is performed with tidal breathing and it requires minimal patient cooperation. However, some young children still have difficulty in staying quiet during the IOS. In these cases, the use of other parameters of spirometry such as forced expiratory volume in 0.5 s (FEV0.5) instead of forced expiratory volume in 1 s (FEV1) could be more effective.