Asthma is the commonest chronic disease of childhood. Whilst most pediatricians have a fairly clear picture of what constitutes asthma, the condition has eluded a universally acceptable, comprehensive and precise definition despite many attempts. It is most simply defined as that state characterised by partial obstruction of airways, reversible with time either spontaneously or as a result of treatment. Clinically it is characterised by episodes of cough, sensations of chest tightness, dyspnoea and wheezing. However, any condition which results in narrowing of intrathoracic airways can present with cough and wheeze. Thus, before launching into the treatment of asthma, it is necessary to consider alternative diagnoses. Whilst the aphorism, "all that wheezes is not asthma" is as true in children as in adults, the commonest errors is failure to diagnose asthma in recurrently wheezy infants. Most alternative diagnoses will be obvious from a clinical history, examination and chest radiograph.