In this dissertation, the design, fabrication and characterization of advanced terahertz (THz) quasi-optical detectors and focal-plane arrays (FPAs) based on monolithically integrated heterostructure backward diodes (HBDs) are presented for THz sensing and imaging applications. In order to develop highly sensitive room temperature THz detectors and FPAs, zero bias Sb-HBDs are attractive to be employed owing to their relatively high curvature coefficient (γ), high sensitivity, low noise equivalent power (NEP), high cut-off frequency, and room temperature operation. To develop high performance detectors and FPAs, HBDs with submicron-scale device areas are preferred for their high detector bandwidth. Since submicron scale HBDs have high device impedances at terahertz frequencies, lens-coupled planar folded dipole antennas (FDAs) which have a wide range impedance tuning capacity at THz frequencies are adopted in the detector design to achieve conjugate impedance matching for maximum detector sensitivities without additional matching networks. For a prototype demonstration, HBDs with 0.16 μm2 and 0.1 μm2 active areas have been employed for the detector design at 200 and 585 GHz respectively. Simulation results show that maximum detector sensitivities of 21,000 V/W at 200 GHz and 9,500 V/W at 585 GHz could be achieved. The corresponding minimum NEPs (NEPsmin) of these detectors are estimated to be 0.42 pW/√Hz and 1.3 pW/√Hz respectively.  In this work, HBDs are integrated with FDAs using submicron-scale airbridges and anode-to-mesa spacing to minimize parasitic capacitance and frequency dependent spreading resistance respectively. On the basis of modeling results, a novel, scalable, and robust fabrication process has been developed using mix-and-match electron beam and optical lithography. A record high zero bias curvature co-efficient of -58V-1 has been obtained for HBDs developed using this process. The measurements of the lens-coupled HBD detector with a 0.7× 0.7 μm2 device area show that a peak detector sensitivity of approximately 2400 V/W and a NEPmin of 2.14 pW/√Hz have been obtained at 170 GHz without applying anti-reflection coating on the silicon lens.If an antireflection coating was used, a sensitivity of approximately 3500 V/W and a NEPmin of 1.48 pW/√Hz are projected. The radiation patterns of the quasi-optical detector in both the E- and H-planes have been measured and good agreement has been achieved between simulation and measurement. Finally, for imaging applications, the single element design has been expanded into full 2D THz FPAs and the off-axis radiation patterns, angular resolution, and mutual coupling have been studied. The reported approach using monolithically integrated heterostructure backward tunneling diodes is promising for developing high performance and compact detectors and FPAs for millimeter-wave and THz sensing and imaging applications.