A selective review on the materials and construction principles used for bifunctional oxygen/air electrodes is given. The discussion emphasizes the catalytically active materials used for the construction of these electrodes, which are a key component in electrically rechargeable air breathing electrochemical systems. Whereas, in acid electrolytes normally noble metal catalysts must be used, there is a possibility to use less expensive transition metal oxides in alkaline electrolytes. Typical transition metal oxides have the perovskite, pyrochlore and spinel structure. (c) 2005 Elsevier B.V. All rights reserved.