In this thesis, I will cover the fabrication, testing, and analysis of microwave kinetic inductance detectors (MKIDs) for both THz and mm-wave spectroscopy. MKIDs are a detector technology that enables large numbers of highly-sensistive mm/submm detectors to be read out using a small number of readout lines, which enables large-format spectroscopy. They can be produced such that a small piece of silicon contains both the spectrometer and detector. The RAxDEx project produced prototype THz spectrometers with a novel, in-chip rectangular waveguide. THz spectroscopy with MKIDs will enable the coupling of large fields-of-view to multi-kilopixel detectors. These devices would drastically improve our ability to map the cooling gas around galactic fountains by using hyperfine cooling lines among other uses for far-infrared spectroscopy. I present prototype dark MKIDs for RaxDEx that match design and noise specifications. In addition, SPT-SLIM is a project to create a mm-wave spectrometer for line intensity mapping with the South Pole Telescope. Line intensity mapping is expected to dramatically increase our knowledge of large scale cosmological structure, particularly in regards to galaxies that are too dim to be detected directly. I will present both dark and optical measurements from SPT-SLIM devices from initial prototypes to the final design submodules. Finally, I will cover an initial analysis of how SPT-SLIM will use unresolved, high-redshift galaxies to constrain the CO power spectrum and the luminosity function.