This article discusses an approach to model the water transport in the membranes of PEM fuel cells during operation. Starting from a frequently utilized equation the various transport mechanisms are analyzed in detail. It is shown that the commonly used approach to simply balance the electro-osmotic drag (EOD) with counter diffusion and/or hydraulic permeation is flawed, and that any net transport of water through the membrane is caused by diffusion. Depending on the effective drag the cathode side of the membrane may experience a lower hydration than the anode side. The effect of a water-uptake layer on the net water transport will also be pictured. Finally, the effect of EOD is visualized using "Newton's cradle". Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.