The outstanding material properties of III-nitride semiconductors has prompted an intense research activity focused on the engineering of resonant tunneling transport within this revolutionary family of wide-bandgap semiconductors. From resonant tunneling diode (RTD) oscillators to quantum cascade lasers (QCLs), nitride devices hold the promise for the realization of high-power ultra-fast sources of terahertz (THz) radiation. Although considerable research effort has been devoted over the past two decades, nitride-based resonant tunneling transport has been demonstrated only during the last four years. In this work, I present the various aspects of heterostructure design, epitaxial growth and device fabrication techniques, which have led to the first unequivocal demonstration of robust resonant tunneling transport and reliable room temperature negative differential conductance in III-Nitride heterostructures. This thesis constitutes a comprehensive work spanning all fronts of experimental, theoretical, and computational research focused on the fundamental physics and engineering of resonant tunneling transport in polar III-nitride semiconductors. Our combined experimental and theoretical approach, allowed us to shed light into the physics of electronic quantum interference phenomena in polar semiconductors which had remained hidden until now, resulting in the discovery of new tunneling features, unique in polar RTDs. The robustness of our experimental data enabled us to track these unique features to the broken inversion symmetry, which generates the built-in spontaneous and piezoelectric polarization fields. After identifying the intimate connection between the polarization fields and the resonant tunneling current, we harness this relationship to develop a completely new approach to measure the magnitude of the internal polarization fields via electron resonant tunneling transport. To get further insight into the asymmetric tunneling injection originated by the polar active region, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current which includes contributions from coherent and sequential tunneling processes is presented. After the application of this new theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities. This agreement allows us to elucidate the role played by the internal polarization fields on the amplitude of the electronic transmission and broadening of the resonant tunneling line shape. Our analytical model is then employed for the design of high-current density GaN/AlN RTDs which are harnessed as the gain elements of the first microwave oscillators and harmonic multipliers driven by III-nitride RTDs. The findings presented here pave the way for the realization of III-Nitride-based high-speed oscillators and quantum cascade lasers that operate at wavelengths that, until now, remain unreachable by other semiconductor materials.