Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention as energy-conversion systems for future applications in vehicles and for on-site power generation. Major technical challenges exist in achieving a high cell performance over a wide range of operating conditions, such as various cell current densities, operating temperatures, and relative humidities of the supplied gases. Correct water management is critical to achieving a high power density, long-term operation, and increased robustness in PEMFCs. Aspects such as the swelling of the membrane by water, the generation and accumulation of liquid water inside the fuel cells, and the discharge of accumulated water need to be clarified to ensure a fundamental understanding of water transport in PEMFCs. In this article, we examine the state of art regarding in situ diagnostics, particularly visualization techniques, for probing the behaviour of water in PEMFCs, with attention to neutron radiography, X-ray imaging, magnetic resonance imaging, and optical visualization techniques. The recent rapid development of in situ imaging techniques with high spatial and temporal resolutions provides a novel platform for the development of PEMFCs. (C) 2010 Elsevier Ltd. All rights reserved.