Characterizing the nature of short-wavelength electromagnetic (EM) field quantities within large complicated enclosures poses a significant challenge in the field of microwave engineering. This problem manifests itself in many situations such as cellular-signal penetration into buildings, specious EM emissions from personal electronic devices inside aircrafts, or the upset of electronic systems due to intentional EM interference threats. In many such cases, the inherent complexity of the enclosure makes a deterministic solution of the internal EM field quantities infeasible, and instead calls for a statistical approach to describe the nature of these quantities. The Random Coupling Model is such a statistical approach and is discussed in this book. The Random Coupling Model treats the enclosure as a wave-chaotic system and predicts "universal" wave-fluctuations whose properties depend only upon the value of a single loss-parameter. The predictions of this model are experimentally validated, and the insights gained suggest design-guidelines for EM-hardening of enclosures to the disruptive effects produced by short-wavelength EM radiation.