A detailed technique is described for the measurement of the permeability of porous media, by the use of either liquids or gases. Derivations are given of the formulae by which the permeability may be computed from the laboratory measurements. In addition to defining a convenient permeability unit, it is proposed to call it the ``darcy.'' Field measurements on the effective permeability of underground sands are discussed, and the appropriate formulae are derived. These include expressions for liquid flow, gas flow, gravity flow and composite artesian and gravity flow. Correction curves are given for application when the wells do not completely penetrate the producing sands.