Recent studies have shown that normal bronchial secretion composed of proteoglycans, atypical glycoproteins and neutral lipids neither includes mucins nor glycolipids, nor phospholipids. The rheological characteristics of bronchial mucus thus depend on mucociliary clearance and clearance of bronchial secretions by cough, which in turn depend on the properties of the glycoprotein acids secreted and on the degree of their entanglement which is linked to their water content and on the chemical bonds with other protein or lipid components which are present in the secretions. Chronic bronchitis, asthma and bronchorrhoea allow for changes in the biochemical composition and the physical and rheological characteristics of the bronchial mucus which alter the clearance. In certain conditions mucus plugs can form. An understanding of the pathology of bronchial mucus in the adult enables one to choose the best therapeutic prescriptions but the efficacy of measurements available remains imperfect.