[en] While routinely achieved performance characteristics of superconducting cavities have now reached a level which makes them useful in large scale applications, achieving this level has come only through the knowledge gained by systematic studies of performance limiting phenomena. Despite the very real progress that has been made, the routine performance of superconducting cavities still falls far short of both the theoretical expectations and the performance of a few exception examples. It is the task of systematically applied diagnostic techniques to reveal additional information concerning the response of superconducting surfaces to applied RF fields. Here recent developments in diagnostic techniques are discussed. 18 references, 12 figures