Assessments of pulmonary function play an integral part in the clinical management of school age children as well as providing objective outcome measures in clinical and epidemiological research studies. Pulmonary function tests (PFTs) can also be undertaken in sleeping infants and in awake young children from 3 years of age. However, the clinical utility of such assessments, which are generally confined to specialist centres, has yet to be established. Whether requesting or undertaking paediatric PFTs, or simply reading about how these tests have been applied in research studies, it is essential to question whether results have been interpreted in a meaningful way. This review summarises some of the issues that need to be considered, including: why the tests are being performed; which tests are most likely to detect the suspected pathophysiology; how often such tests should be repeated; whether results are likely to be reliable (in terms of data quality, repeatability and the availability of suitable reference equations with which to distinguish the effects of disease from those of growth and development), and whether the selected tests are likely to be feasible in the individual child or study group under investigation.