A kinetic theory of permeability is developed both for the case where the cell membrane is a distinct phase and where it is simply the interface between cell and environment. A general expression for the diffusion current of a substance across the membrane is derived, which depends on two “coefficients of permeability” rather than one. Solubility and adsorption theories of permeability are shown to be special cases of the formulas. The permeability coefficients are expressed in terms of the difference of potential energy of the diffusing substance in the two phases; for a membrane of finite thickness they also involve the thickness, and the diffusion coefficient of the solute in the membrane.