Muco-ciliary transport is only effective because of the coordination of the ciliary beats (metachronous) and the harmony between mucus and cilia. The tip of the cilia is in contact with a jellyform layer of mucus propelled to the oropharynx. This jellyform layer has a complex rheological behaviour: it flows like a liquid and shapes like solid elastic. When the rheological properties of bronchial secretion are abnormal, mucociliary transport becomes inefficient. However, the most fluid secretions are not necessarily best transported, because the elasticity and viscosity to guarantee efficient muco-ciliary transport can only vary within defined limits. The mechanism regulating the ciliary beats is poorly understood; the bronchial secretions conduct impulses through the autonomic nervous system as well as mediators such as histamine and the metabolites of arachidonic acid. Mucociliary function may be studied either, directly through mucociliary transport or through mucociliary clearance. A fall in mucociliary activity can be produced by a primary ciliary disorder, by bronchial disease or the consequences of respiratory infection. General anaesthetics and Atropine slow mucociliary transport but Ipratropium bromide does not; Theophylline and sympathomimetics speed it up. The expectorants are mucolytics (proteolytic enzymes, N-acetyl-cysteine), there are agents to correct hydration anomalies of the bronchial secretion (water, hypertonic sodium chloride) iodides, antifibrins by substitution, anti-inflammatory agents and mucoregulatory agents (S-carboxymethylcysteine, bromhexine). The efficacy of the greater part of these expectorants has not been established in vivo by controlled therapeutic trials.