Recent developments in solar watersplitting photocatalysis. This issue of MRS Bulletin focuses on the principles of solar water splitting and highlights recent developments with semiconductor light absorbers and co-catalysts. The cover image conceptually represents the process of converting water to H2 and O2 molecules with solar energy driving a metal oxide photocatalyst. For high effi ciency, the catalyst needs to absorb visible light, generate electron-hole-pairs of sufficient energy, and provide reaction sites for water electrolysis at low overpotential. Materials that combine all of these characteristics and that remain chemically stable over long periods of time do not exist today. The graphic was created by Erwin Sabio, Department of Chemistry, University of California, Davis. See the technical theme that begins on p. 17.