Solar cells based on the perovskite-structured light absorber CH3NH3PbI3 have recently emerged at the forefront of solution-processable photovoltaic devices, with power conversion efficiencies as high as 20.1% having now been certified. Despite this rapid progress, the instability of CH3NH3PbI3 with respect to moisture remains the single biggest obstacle to the commercialization of these devices. In the presence of high levels of humidity, the dark brown perovskite undergoes rapid decomposition into lead iodide, resulting in a significant decline in device performance. Additionally, since PbI2 is partially soluble in water (ca. 250 ppm Pb at 25 °C), this decomposition process presents an enormous ecotoxicological concern. In this presentation, I will discuss our research group’s recent efforts to elucidate the decomposition mechanism of CH3NH3PbI3, both in the presence of moisture and at elevated temperatures.