The respiratory epithelium comprises a mixed population of ciliated, nonciliated, and mucous-secreting cells from proximal to distal airways. The individual characteristics of these cells create not only an effective physical barrier against various noxious substances, but also a highly sophisticated host defense system by producing and releasing a large number of chemical mediators and cytokines. The bronchial epithelium consists of both, the surface epithelial cells and mucus glands. The surface epithelial cells comprise three principle cell types; basal, goblet, and ciliated cells, of which the latter two form a suprabasal columnar structure. They are necessary for mucociliary clearance. Studies using human bronchial epithelial cells (BEpiC) have demonstrated that IL-4 and IL-13 stimulation can modify cellular proliferation, but also ciliary beating, and mucous production. BEpiC proliferation depends in part by EGF receptor signaling. Cultured BEpiC are a useful in vitro model for studying the function and pathophysiology of the bronchial epithelium.