In this article a review is presented of the morphology and function of respiratory cilia and emphasis is placed on the importance of mucociliary clearance as the most important defense mechanism of the upper and lower airways. Physical factors and pharmacological substances which can influence ciliary activity and mucociliary transport are mentioned. Finally, a description is given of changes, mostly reversible, of the mucociliary transport system in infections and IgE-mediated allergy and of the, irreversible changes in congenital diseases like cystic fibrosis and primary ciliary dyskinesia, with some remarks as to the therapeutical consequences of these disturbances.