Knowledge of the gas flow in pumping lines, in internal channels, and in porous materials undergoing vacuum processes is basic to the design and operation of a vacuum system. The gas flow equations for viscous and molecular flow in cylindrical channels are ideally simple, in slip flow they are not so easy, but in porous materials in all the flow regions they are more complicated. The permeability of a porous material has been obtained directly by measuring with a flowmeter the amount of gas flowing through it under an absolute pressure gradient, and the same has been done with model materials. Correlation of the permeability and mean absolute pressure provides basic information on structural factors of the material such as porosity, tortuosity factor, and equivalent pore diameter which could, for example, be applied to vacuum and freeze-drying processes.