This chapter shows why at least the contact zone of a pneumatic seal must be substantially different from that of a hydraulic seal. In many applications, oil-mist lubrication of pneumatic equipment is no longer permissible for environmental reasons. The answer is to use oil-free air and provide a once-for-all lubricant film that remains intact for the life of the equipment. The elastomeric lip of a typical pneumatic piston seal; the initial sealing contact stress is determined by interference between seal and countersurface. The development of modern pneumatic seals was governed by the need to establish a permanent lubricant film that is adherent and protects against corrosion. Pneumatic seals with a compact cross section in radial compression, or a radially thick lip, are sensitive to manufacturing tolerances. This gives rise to appreciable friction differences between seals.