Airway inflammation is important in the pathogenesis of asthma, during which it may lead to symptomatic exacerbations and increases in asthma severity, as well as contribute to future decline in asthma status. The use of induced sputum has emerged as an important and useful technique to study airway inflammation. It has particular advantages in the study of childhood asthma because it is noninvasive and allows samples to be collected on repeated occasions in children over 7 years of age. The results of cell counts are reliable when the sputum is processed in a standardized manner involving selection from saliva, cell dispersion, and quantitative cytology. Children with asthma have increased eosinophils and mast cells, which may persist even with high doses of inhaled corticosteroid therapy. During a severe exacerbation of asthma, there is an intense and heterogeneous inflammatory response involving eosinophil and neutrophil accumulation and activation. Characterization of the relevance of airway inflammation in children with asthma is important.