Bronchoalveolar lavage (BAL) greatly improved our understanding of asthma allowing to demonstrate the key role of inflammation in the pathogenesis of the disease. BAL is a safe procedure, even in severe patients when properly performed. BAL samples large and small airways and alveoli. Cells and mediators may be measured in BALF but they only represent an indirect estimation of the bronchial inflammation. Before performing BAL, the clinical status of the patients should be ascertained and drugs taken may have to be withdrawn. BALF markers should follow some requirements: (1) markers should be released by cells that are pertinent to airways inflammation (and reparation) in asthma, and, if possible they should be specific of a single cell type, (2) the enumeration of cells or titration of the marker or of its metabolites should be specific and sensitive, (3) if possible the titration should not be modified by the sampling procedure, (4) pilot studies should have demonstrated that the cell is increased or the secretory product is released during challenge in asthmatic subjects, (5) studies in a large number of patients should have demonstrated that the levels of the marker are increased in chronic asthmatics, that these levels are correlated with the severity of the disease and are decreased during effective anti-inflammatory treatment, and (6) if possible the cell or marker should be specific to asthma (but at present there is no such cell or marker). Eosinophils and granule secretory products follow most of these requirements. BAL represents an important research tool to assess the effects of therapeutic interventions.