Due to the extremely narrow bandwith of a superconducting cavity around its resonant frequency in the fundamental mould, all techniques for measuring the Q factor, valid for a normal conductive cavity, cannot be straightforwardly applied to the measurement of a Q factor of the order of 1.0exp 10, i.e. with a bandwith below 1Hz at a frequency of a GHz. The decay method will be explained and, with it, the working principles of the main instrumentation like circulators, pin diode modulator, power heads, spectrometers, and so on.