Permeability is defined here from the equations of fluid flow through porous media. It represents, for composite reinforcements, a measure for their ease of impregnation by a fluid resin. Modeling of the saturated permeability tensor is presented using a historical perspective, going from the first geometrical models developed in soil science, to recent unit-cell models based on a precise description of composite reinforcements. Deviations from the saturated flow models are then presented, discussing in particular the effect of progressive saturation. Finally, experimental methods to measure permeability are described and discussed.