The Center for Disease Control and Prevention (CDC) released heat illness data which highlighted that ~29 heat stress hospitalizations and ~3 heat-related deaths occurred every day during the summer months within the US from years 2000 to 2014. Heatstroke- the most severe form of heat illness which oftentimes lead to death- has been cited to be entirely preventable if a timely intervention is introduced. This dissertation uses microwave radiometric thermometry to perform wireless non-invasive internal body temperature monitoring which can enable intervention methods that help to prevent deaths associated with heat-illness. Overall, this dissertation develops a comprehensive closed-form analytical radiometric model and validates the effectiveness of the comprehensive model through a controlled life-like human body temperature sensing experiment. Wireless sub-skin temperature data is predicted from a human tissue mimicking phantom testbed to within 1%. A generic isolated radiometer system equation is derived for all possible calibration source combinations. The generic isolated radiometer system equation predicts comparable results to that of an ideal simulation. While improved isolation decreases measurement uncertainty, it does not improve the accuracy of estimated noise temperatures using a perfectly-isolated radiometer system equation assumption. A highly reproducible tissue-mimicking biological phantom (bio-phantom) recipe (comprised of urethane, graphite powder, and a solvent) was developed to accurately emulate the electrical properties of actual dry human skin versus frequency up to 18 GHz. The developed solid state skin phantom begins in pourable liquid form and then cures at room temperature into a dry solid state mold. An in-plane electromagnetic bandgap structure was developed and integrated within an on-body inward facing spiral antenna design. The inclusion of the in-plane electromagnetic bandgap structure demonstrated a +2.64dB gain improvement in the antenna broadside and -8dB in the rear gain while in-contact with the body as compared to the conventional spiral antenna. Likewise, the measured main beam efficiency is improved from 54.43% for the conventional antenna to 86.36% for the EBG antenna. Two techniques based on signal-flow graph theory were derived to explain both the non-coherent steady-state radiative transfer and the coherent radiative transfer within multi-layered dielectric media with non-uniform temperatures and any number of stratified layers. Both models allow for the accurate characterization and sensing of the thermal emissions originating from subsurface tissue layers.