Pressure-driven flow through porous media is a well-investigated subject of fluid and gas dynamics. Since aerogels possess a nanostructure and porosities above 90%, the flow through the pores needs special consideration. We only discussgas flow through aerogels. First, there is of course the conventional viscous flow determined mainly by the pressure gradient and the viscosity, as in Hagen–Poisseuille flow. In such a flow situation, the molecules interact with each other more frequently than with pore walls. Knudsen flow is determined by the interaction of molecules with pore walls, meaning collision events between themselves are negligible. The third possibility is a sliding of molecules along the surface of the pore walls determined by the friction coefficient between molecules and the pore surface. The essential characteristic property determining the flow through a porous body is the so-called permeability. The chapter derives not only the basic flow equations for porous mediabut also discusses experimental approaches to determine gas phase permeability and compare experimental results with theoretical models.