Metal halide perovskite solar cells (PSCs) have revolutionized the field of thin film photovoltaics. Within a decade, the power conversion efficiencies (PCEs) have increased at a phenomenal rate, rising from 3.8% to more than 25% in single-junction devices, moving them ahead of the current silicon-based technology. The high efficiencies of perovskite solar cells (PSCs) and their other unique properties arise from a combination of organic and inorganic components and electronic-ionic conduction, making them excellent candidates for a plethora of applications. However, PSCs face a significant—and ironic—roadblock to commercialization: these light-harvesting materials degrade under sunlight—the very condition they would need to endure to realize their potential in real-world devices. Thus, my research focuses on identifying the origin of this degradation and exploring viable pathways for its mitigation. Our hypothesis, based on results from our lab and literature, is that ion migration under illumination causes the instability in metal halide perovskites. The first half of my research focuses on identifying the nature of mobile ion(s), which conditions cause them to move, and how they move through the material. The second half is centered on approaches to rationally design perovskite compositions and device interfaces to suppress ion migration. As a result, we were able to fabricate highly stable and efficient PSCs. For example, through large cation substitution in MAPbI3 and by introducing bilayer hole transport layer, we were able to fabricate inverted (p-i-n) devices with an average PCE of 19%, and a maximum PCE above 20% with over > 4500 h stability under illumination. To the best of our knowledge, this is among the highest reported efficiencies for inverted PSCs.