The middle ear is an organ that, like the lung, must maintain an aerated cavity within it to carry out its function. When hypoventilation occurs in the lung it threatens the maintenance of life, when it occurs in the middle ear it causes loss of hearing. For the middle ear to fulfill its vital sound-conduction function, its pressure must be maintained at approximately atmospheric level. The eustachian tube has long been known to play a key role in pressure regulation, but later research has revealed that transmucosal gas exchange is as important as the function of the eustachian tube, and that the ventilatory physiology of the middle ear is maintained by these two mechanisms in tandem. The findings presented in this volume answer many questions about ventilation and pressure regulation of the middle ear, and should be valuable to clinical practitioners in choosing appropriate modes of treatment for pathological conditions such as cholesteatoma and otitis media.