The Intelligence Community Studies Board of the National Academies of Sciences, Engineering, and Medicine convened a 2-day virtual workshop on December 17 and 21, 2021, to explore insights from world-class experts and technologists familiar with the extensive range of issues associated with anticipating rare events--those characterized by a very low probability of occurring--of major significance. Over the course of the 2-day workshop, the speakers discussed analytical methods, computational advances, data sources, and risk assessment approaches for anticipating rare events, including natural disasters, pandemics, anthropogenic threats, and widespread technological change. This proceedings is a factual summary of the presentations and discussion of the workshop.