One of the major problems in pediatric pulmonology has been the shortage of tools to assess objectively the mechanical status of the lung and the effects of therapy. In the last few years this has changed; there are now a large number of methodologies to choose from. One method has rarely been tested against another, and as there does not appear to be a gold standard, the situation is becoming confused. Choice of the technique to be applied must be based on the characteristics of the patient to be studied. With the recent introduction of commercially available devices for performing some of these techniques, it is essential that those using them have a clear understanding of what they are measuring, the limitations of the measurements, and their interpretation.