In the 1980s, studies in the vascular field revealed that the endothelium was not simply a metabolic and physical barrier, but liberated substances that could modulate the function of underlying vascular smooth muscle. Investigators in the respiratory field also found that the airway epithelium was more than a physical barrier to airborne insults. The epithelium is composed of at least eight different cell types that have a range of functions, including ciliary motility and mucous secretion, and contain enzymes for liberating arachidonic acid metabolites and peptides. The epithelium also contains degradative enzymes for a number of peptides and biological amines. It was also recognized that the epithelium released substances that, like their vascular counterparts, could regulate the function of a number of cell types, including nerves and airway smooth muscle. These studies document the importance the epithelium plays in the regulation of human airway smooth muscle.