There is a growing awareness that hydrogen is the fuel of the future. While hydrogen can be generated using different technologies, only some of them are environmentally friendly. It is argued that hydrogen generated from water using solar energy, solar-hydrogen, is a leading candidate for a renewable and environmentally safe energy carrier due to the following reasons: Solar-hydrogen technology is relatively simple and, therefore, the cost of such a fuel is expected to be substantially less than that of the present price of gasoline. The only raw material for the production of solar-hydrogen is water, which is a renewable resource. Large areas of the globe have ready access to solar energy which is the only required energy source for solar-hydrogen generation. The development of solar-hydrogen technology requires new photo-sensitive materials serving as photo-electrodes in electrochemical devices that convert solar energy into chemical energy (hydrogen). As photo-electrodes are likely to be made of inexpensive polycrystalline materials rather than expensive single crystals, it is important to realize that the photo-sensitivity of polycrystalline materials is strongly influenced, if not determined, by the local properties of interfaces, such as external surfaces and grain boundaries. Consequently, the successful development of novel photo-sensitive materials will be determined by progress in the science and engineering of materials interfaces. There is also a need to increase the present state of understanding of the local properties of interfaces, such as defect disorder, electronic structure, and related semiconducting properties, on the impact of interfaces on photo-electrochemical properties. The present paper briefly outlines the main challenges in the development of materials for solar-hydrogen. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.