Metal halide perovskites are an attractive material for a wide variety of optoelectronic applications, from solar cells (photovoltaics) to radiation detection. This is primarily due to their ease of solution processing, strong and tunable light absorption, and high tolerance to surface defects. These same properties make metal halide perovskites attractive for photocatalytic applications as well. The dissertation research described herein presents a body of work investigating the fundamental excited state interactions of perovskites - those processes that occur after absorption of visible photons. In the context of photocatalysis, interfacial electron transfer, energy transfer, and surface chemistry considerations are discussed. In the context of photovoltaics, insights into the process of light-induced phase segregation are presented. Understanding of these fundamental excited state processes is key to developing better perovskite materials with improved properties.