© The Royal Society of Chemistry. Oxynitrides of transition metals, alkaline earth metals and rare earth metals are intensively investigated as a group of materials to expand and tune the properties of oxides. The differences in polarizability, electronegativity and anion charge of nitrogen and oxygen induce changes in the physical and chemical properties of oxides by nitrogen introduction. The effects on properties arise from the higher covalency of the metal-nitrogen bond and the changes in the energies of electronic levels, and are important in slightly doped nitrogen metal oxides as in stoichiometric oxynitrides. More intense recent progress in oxynitride research has been made in some specific fields such as photocatalysis in water splitting and other processes as the observed small band gaps lead to activity in the visible light range. The stabilization of new perovskite oxynitrides, with the oxidation states of cations tuned by N/O stoichiometry, has led to new magnetic and dielectric materials. The lower electronegativity of nitrogen and larger crystal field splitting induced by N3- shifts the emission wavelengths of phosphors to the red, and oxynitridosilicates have been investigated as components of white LEDs.