The definition of asthma in young children is clinical, supplemented in older children by physiological measurements to define variable airflow obstruction. Understanding of the underlying processes, and hence best treatment, may be aided by defining different phenotypes on the basis of the components of inflammation, bronchial hyperreactivity and any persistent airflow limitation. In the preschool child, a noninflammatory phenotype, devoid of bronchial hyperreactivity, is characteristic of virus-associated wheeze and postbronchiolitic cough and wheeze. Chronic lung disease of prematurity is characterized by peak flow variability and persistent airflow limitation, whereas obliterative bronchiolitis is a pure form of fixed airflow obstruction. Atopic wheeze is likely a combination of inflammation and reactivity, as in the older child, although firm data are lacking. In the older child, a number of inflammatory and noninflammatory phenotypes are described, including (high dose) steroid sensitive asthma; persistent eosinophilic inflammation, with and without symptoms; neutrophilic inflammation; noninflammatory bronchial reactivity; obliterative bronchiolitis; overestimation of symptoms; and poor concordance with therapy. These concepts have been largely derived with invasive studies, which would be inappropriate to milder forms of asthma. Nonetheless, milder asthma can be assessed by the same protocols using noninvasive techniques. However, further multicenter, probably multinational, studies are needed to validate this approach.