After recalling the characteristic properties of a superconductor, the authors present theoretical and experimental results concerning the microwave surface resistance and upper-limit values of the radio frequency fields in a resonant cavity. Details are also given on technological processes used for the fabrication of resonators with high-quality metal surfaces. A great part of the paper is then devoted to the most important field of applications of superconducting resonators: the frequency stabilisation of microwave generators for metrological purposes, and the realisation of electron (or ion) continuous-wave linear accelerators. Other applications also based upon a very low surface resistance are briefly reviewed. Microwave Josephson devices are not considered.