Why use microwaves to measure fundamental properties of superconductors? To understand the answer, one must consider that at microwave frequencies superconductors have finite resistance. The resistance is identically zero at dc and negligible at very low frequencies, so the superconductor does not interact with such probes. The origins of the finite resistance will be explained in Section 8.1.2. Microwaves interact strongly, and over the history of superconductivity microwave measurements have contributed considerable information to the understanding of the phenomenon. In the very early history of superconductivity, microwave technology was not developed enough to be of use in experiments of the fundamental properties. So it took the development of both the two-fluid model to illustrate the interaction of microwaves and the development of microwave technology immediately before and during World War II to make the realization of the microwave experiments possible. In this section, I use the broadest definition of microwave frequency range, from approximately 300 MHz to 300 GHz. However most of the experiments described here are in the range 1 to 20 GHz. In the microwave frequency range, one determines the resistance by measuring the quality factor Q of a microwave resonator and one determines the penetration depth by measuring the reactance of the resonator. More detail is presented below.