Water management is one of the most important issues in Proton Exchange Membrane Fuel Cells (PEMFC). Flooding and drying out are the two main degradation mechanisms that occur when water management is not adequate. This paper overviews the underlying phenomena linked to water management and some characterization methods or strategies to prevent their occurrence. Finally, a fault tree is built, in order to discriminate the different defaults and better understand the relationship between the causes and symptoms. (C) 2008 Elsevier B.V. All rights reserved.