The present work considers unresolved problems that need to be addressed for the development of photo-electrodes for photo-electrochemical generation of hydrogen from water using solar energy. The present work is focused on photoelectrodes based on oxide semiconductors, including TiO2 and TiO2-based materials, which are the most promising candidates for photo-electrodes. The properties of photo-sensitive oxide semiconductors are considered in terms of the concepts of photo-electrochemistry and the concepts of defect chemistry. The role of defect chemistry as a framework for the development of photo-electrode is considered. The effect of defect disorder on the performance-related properties of TiO2-based photo-electrodes are discussed. The major challenges in the development of materials with optimized performance are considered in terms of both collective and local properties that must be assessed and modified simultaneously. (c) 2005 Elsevier Ltd. All rights reserved.