Degradation is one of the most important issues in solid oxide electrolysis cells (SOECs). The lowest long-term degradation reported in the literature for SOEC under practical operation is about 1.7%/1000 h during 3600 h at 1 A/cm(2) which is twice the degradation of similar cells under fuel cell operation. To reduce the degradation, a clearer identification of the polarization and degradation mechanisms of SOEC is important in order to adapt the operating conditions, optimize the cell architecture to the purpose of electrolysis by modifying and/or replacing one or both of the electrodes with alternative materials. This paper tends to review the cell degradation phenomena observed under high temperature electrolysis. A description of reactions potentially involved in the degradation mechanisms associated with the thermodynamic and kinetic considerations are used to build a fault tree in order to classify the different defaults and better understand the relationship between the causes and symptoms. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.