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.Table of ContentsFront Matter1 Introduction and Overview2 Mathematical Foundations for Anticipating Rare Events3 Detection, Indications, and Warnings4 Planning, Forecasting, and Intelligence Preparation5 Fireside Chat - Using Artificial Intelligence to Predict the Occurrence of Sepsis6 Fireside Chat - Rare Events and Insurance7 Multisource Information Fusion, Situation Assessment, and Course of Action Selection8 Active Prevention and Deterrence9 Concluding RemarksAppendixesAppendix A: Statement of TaskAppendix B: Workshop AgendaAppendix C: Planning Committee BiographiesAppendix D: Speaker Biographies