The performance of proton exchange membrane fuel cells (PEMFCs) is significantly affected by the presence of liquid water in the flow-field channels and gas-diffusion electrodes and dissolved water within the membrane. Quantification of water levels would lead to a better fundamental understanding of water management issues, provide links with performance loss, and more fully validate mathematical model predictions. These beneficial outcomes have recently and significantly increased development efforts, resulting in a present inventory of 10 main measurement methods. A focused method-development roadmap is proposed based on ideal method requirements to reduce cost and accelerate activities. (C) 2007 The Electrochemical Society.