This book focuses on water content estimation and control of the PEM fuel cell stack and the individual cell in vehicle. Firstly, the mathematical connection between polarization curve and equivalent circuit model proves importance of MEA and its feasibility to study water content. Optimizing structure of MEA realizes the internal water content recirculation of a fuel cell and improves its performance under middle or lower current density. The influence of water content on performance of MEA is quantified, and variation of equivalent circuit model is an excellent indicator of water...