Whispering Gallery Mode Resonators (WGMRs) are well known high-Q resonators especially for their application as label-free biomolecular sensors for medical applications. Here the interaction of confined whispering gallery electric field modes with the analytes paves the way to high sensitivity detection in real time and in a label-free style at optical frequency. On the other hand, the growing demand for expanding terahertz (THz) wave technology to unravel ’THz gap’ issue is particularly important to be addressed and it has been resolved so far by building active THz elements like THz spectrometer, THz meta material devices, THz WGMR, to mention a few. In this talk, I will briefly discuss the development of the THz sensor project I have been part of and also discuss the theoretical and experimental analysis of the proximity dependence of spherical whispering gallery mode and the THz waveguide mode based on coupled mode theory (CMT) and beam-splitter (BS) model. The distance dependent broadening of the lorentzian WG modes established our understanding on coupling interaction strength between the modes. This helped us to understand the overcoupling, critical coupling, and undercoupling regimes of the interactions involved. Based on the above findings, we successfully optimized the Q-factor of the THz WGM component experimentally to ensure high-Q and an application based on refractometric sensor with high sensitivity has been reported. Here we evaluated the absorption coefficient of the adsorbed molecule on the WGM device based on the analytical approach rooted on coupled mode theory between the waveguide mode and the spherical resonator mode. The model has been modified to evaluate optical constants of materials. The results obtained have been verified by continuous wave THz transmission results. The results are of importance in sensing, metrology and material characterization.