Why are the small airways important in the pathology and treatment of asthma? Asthma is an inflammatory disorder of the airways often associated with structural abnormalities and progressive decline in lung function. Successful therapy therefore requires efficacious drug delivery to these airways. Traditionally, inhaled asthma therapies have mainly targeted the large airways of the lung. There is increasing evidence, however, of inflammation and chronic structural changes within the small airways in patients with asthma, and thus it is pertinent that the therapeutic targets for inhaled corticosteroids are reconsidered to take into account all of the pulmonary tissues that may respond to inhaled anti-inflammatory treatment. The challenge now lies in evaluating the value of the small airways as therapeutic targets in asthma. The articles in this supplement critically assess the evidence for inflammatory and structural changes in the airways of patients with asthma, recent technical advances in the monitoring of inflammation, and the clinical efficacy of topical corticosteroids and how this may relate to their ability to target the small airways. As the small airways normally contribute only about 10% of airway resistance (the ‘quiet zone’), the difficulty of in riro sampling and physiological measurements that are specific for this site has led to their considerable under-evaluation (I). However, techniques such as transbronchial biopsy, along with the analysis of resected lung tissue and post-mortem tissue, have demonstrated that extensive disease exists in the small airways. Airway remodelling, potentially leading to a component of fixed airflow obstruction, has also been shown to occur in the small airways (2). Fibroblast proliferation and collagen deposition may also contribute to this permanent tissue damage (3,4). In contrast to conventional pulmonary function tests, which lack specificity for small airways, recent advances in high-resolution computed tomography imaging have allowed non-invasive reproducible measurements of structure-function relationships in the small airways and in viva assessment of drug deposition (5,6). This method, although providing indirect data of air trapping and