Abstract The application of photoelectrochemical systems based on photoactive semiconducting electrodes to the problem of solar energy conversion and chemical synthesis is discussed. Three types of cells are described: electrochemical photovoltaic cells (wherein optical energy is converted into electrical energy); photoelectrolysis cells (wherein optical energy is converted into chemical free energy); and photocatalytic cells (wherein optical energy provides the activation energy for exoergic chemical reactions). The critical semiconductor electrode properties for these cells are the band gap, the flat-band potential, and photoelectrochemical stability. No semiconductor electrode material is yet known for which all three parameters are simultaneously optimized. An interesting configurational variation of photoelectrolysis cells, labelled ‘photochemical diodes’, is described. These diodes comprise cells that have been collapsed into monolithic particles containing no external wires. Recent advances in several areas of photoelectrochemical systems are also described.