Nanomaterials for electrochemical water splitting. Water-splitting electrolysis using an electrocatalyst and a renewable power source is a promising energy-conversion technology, especially when combined with energy stored in the form of hydrogen that has the benefi t of also being environmentally friendly. The electrocatalyst can be rationally designed using nanomaterials spanning from transition-metalbased oxides and their derivatives, organic polymer nanomaterials, to inorganic–organic nanocomposites. This issue of MRS Bulletin discusses materials innovations for realizing highly effi cient and durable electrocatalysts for large-scale, cost-effective water splitting. On the cover is a solar cell (with an electrocatalyst) that generates oxygen molecules (red) on the left electrode and hydrogen molecules (green) on the right electrode. See the technical theme that begins on p. 531.