Book excerpt: These lectures are concerned with the application of high temperature superconductors to both passive and active high-frequency devices.The central issue addressed is the electrodynamics of granular superconductors, particularly where grain boundaries (either natural or synthetic) act as Josephson weak-links. Grain boundaries are responsible for residual dissipation and for unwanted dependence of the electromagnetic properties on ambient magnetic fields and on elevated power level. Properly controlled, similar weak-links are the key to high sensitivity dc and rf SQUIDS at readily accessible temperatures, and to modulators, mixers and detectors. Such structures may conveniently lead to superconductive electronic devices as well as coherent sources of radiation in the very far infrared.