Proton exchange membrane (PEM) water electrolysis offers several advantages compared to the traditional alkaline technologies including higher energy efficiencies, considerably higher specific production rates leading to more compact design, and avoiding a liquid and highly corrosive electrolyte. The oxygen electrode is the critical part in the energy consumption of such cells. To obtain high performance, electrocatalytically very active anode materials have to be developed for the oxygen evolving interface. The most promising electrocatalytic materials are based on IrO2 and RuO2, preferably in mixtures with other transition metal oxides with electronic conductivity. Excellent performance has been obtained by using nanocrystalline electrocatalysts based on iridium oxide with additions of ruthenium oxide and/or tin oxide, forming rutile structures, or mixed with tantalum pentoxide.This concept has been applied extensively in our work and has been successful in understanding oxygen evolution performance variations in IrO2-RuO2, IrO2-SnO2, IrO2-RuO2-SnO2 and IrO2-RuO2- Ta2O5 systems.