High-power terahertz quantum-cascade lasers (QCLs) are desired for a variety of applications in imaging and spectroscopy. The best performance at practical operating temperatures for single-mode terahertz QCLs is realized with metallic cavities due to a strong plasmonic mode confinement of the optical mode within the cavity. However, such plasmonic lasers suffer from poor beam shapes, low output power, and multi-mode spectral behavior. Development of distributed-feedback (DFB) techniques to improve spectral as well as modal properties becomes indispensable for terahertz QCLs to address targeted applications that typically require single-mode operation, frequency stability and specificity, and optimal far-field beam quality with single-lobed profile and low angular divergence. This chapter describes the theory, design methodologies, and key results from a sampling of a wide variety of DFB techniques that have been implemented in literature for monolithic terahertz QCLs with metallic cavities in both edge-emitting and surface-emitting configurations, either of which have their specific application areas and advantages much-like that for infrared diode lasers.