High temperature sensors based on solid electrolytes are well established in many applications. In this context. stabilized zirconia as an oxygen ion conductor plays the key role to monitor oxygen. Recent trends in the use of stabilized zirconia aim at the detection of other exhaust or environmental gases like NOx, CO or hydrocarbons. This is possible by, e.g. monitoring currents of specific electrode reactions at a given voltage ('amperometric devices') or monitoring voltages between different electrodes which result from specific electrode reactions including the kinetically determined formation of surface oxygen during different catalytic reactions ['mixed-potential (non-Nernstian) devices']. In all applications, the catalytic and electrochemical properties of the electrode materials play a key role for achieving reproducible sensor signals. This requires controlled microstructures down to the atomic scale. Therefore, extensive work now focuses on the structural as well as functional optimization of known, and on the development of new electrode materials. In this promising trend particular emphasis is put on the in-situ spectroscopic and microscopic characterization of the various interfaces under sensor operation conditions.