The Cosmology Large Angular Scale Surveyor (CLASS) is an array of polarization-sensitive millimeter-wave telescopes that observes ~70% of the sky in frequency bands centered near 40 GHz, 90 GHz, 150 GHz, and 220 GHz from a high-altitude site in the Atacama desert of northern Chile. It seeks to measure polarization anisotropy in the cosmic microwave background (CMB), with a particular emphasis on measuring the optical depth due to reionization via large-angular-scale polarization E-modes, as well as searching for primordial polarization B-modes, a detection of which would provide strong evidence for cosmological inflation. This dissertation starts by providing an overview of physical cosmology, before describing the science goals and instrument design of CLASS. It then describes various instrument components that were developed, describes a novel 3D-printed millimeter-wave absorber, and describes the control and systems software used to operate the telescopes. Analysis efforts are then covered, specifically the modeling and detection of atmospheric circular polarization due to Zeeman-splitting of molecular oxygen emission lines in the geomagnetic field and a method of cleaning CMB foregrounds from full-sky maps that utilizes machine learning techniques
