How does online extremism cross over into the offline world and cause physical harm and violence? This is one of the most difficult questions to answer about online extremism. We know that its online harm is pervasive. Extremists commonly use—and abuse—social and digital media technologies and spaces to express hateful speech, as well as target specific individuals with this speech. Extremists also can amplify their harmful speech by convincing many others on a platform to share their messages or post original expressions of support. We additionally know that online extremism can help incite physical violence, such as the mosque shootings in New Zealand, several attacks in European cities by sympathizers of the Islamic State, and the January 6, 2021, riot at the US Capitol Building. But these latter kinds of harm—physical instances of mass violence and unrest—are rare, especially compared to the number of people who generate and consume extremist content online. Herein lies the difficulty: It is hard to identify reoccurring, generalized processes of how things work when only analyzing rare events.