Airway inflammation is a characteristic feature of asthma. Bronchoscopy with bronchoalveolar lavage (BAL) and/or mucosal airway biopsies has provided invaluable information about the nature of asthmatic inflammation. A common finding in all BAL studies in asthma is increased numbers or proportions of eosinophils even in subjects with mild disease. BAL cells in asthmatics demonstrate a cytokine profile consistent with a T helper2-like phenotype, with increased expression of interleukin (IL)-4 and IL-5. Endobronchial mucosal biopsies show loss of surface epithelium, thickening of the reticular layer of the basement membrane and an increased cellular infiltrate of mainly eosinophils, mast cells and T lymphocytes, and up-regulation of a number of cytokines. Recently, peripheral airways from asthmatics have been investigated, and even more intense inflammation has been described. However, in severe steroid-dependent asthmatics, intriguing differences in the type of inflammation have been described. The term 'airway remodelling' indicates structural changes seen in long standing asthma, where myofibroblasts might be of particular importance. BAL and mucosal biopsies are important tools in the investigation of airway inflammation in asthma.