Airway inflammation is fundamental to the cause and persistence of asthma and other airway conditions. It contributes to symptoms, variable airflow limitation, airway hyperresponsiveness, and the structural changes (remodeling) associated with asthma. However, the presence and type of airway inflammation can be difficult to detect clinically, delaying the introduction of appropriate treatment. Cellular inflammation in the airway can be accurately and reliably assessed by examining spontaneous or, when not available, induced sputum. Induced sputum cell counts are relatively noninvasive, safe, and reliable. They can accurately discriminate eosinophilic airway inflammation from noneosinophilic airway inflammation and, thus, help to guide therapy. Eosinophilic airway inflammation is steroid responsive, whereas noneosinophilic (usually neutrophilic) inflammation generally is not. Monitoring of airway inflammation using sputum cell counts helps to identify the impending loss of asthma control and, thus, the need to adjust antiinflammatory medications in patients with a variety of airway diseases, such as asthma, smoker’s COPD, and chronic cough. Other noninvasive, indirect measurements of airway inflammation, such as exhaled nitric oxide, do not help to identify the cellular nature of airway inflammation associated with exacerbations of airway diseases, particularly in patients who are already on corticosteroids. Thus, although they can be a predictor of steroid responsiveness, these measures do not help to reduce asthma exacerbations when used in clinical practice. The clinical usefulness of measurements in exhaled breath condensate has not yet been established.