Asthma is the commonest chronic disease in childhood. Due to the various different phenotypes of childhood asthma, it has been difficult to agree on a clear definition of the condition and instead an operational description is used: ‘Asthma is a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role. The chronic inflammation is associated with airway hyperresponsiveness that leads to recurrent episodes of wheezing, breathlessness, chest tightness, and coughing, particularly at night or in the early morning. These episodes are usually associated with widespread, but variable, airflow obstruction within the lung that is often reversible either spontaneously or with treatment.’ (Global Initiative for Asthma, 2012). However, in children <5 years of age, clinical symptoms of asthma are variable and nonspecific, and a symptoms-only approach that defines various wheezing phenotypes has been recommended.