In this paper, I review U.S. constitutional considerations for lawmakers seeking to balance terrorist threats against free expression online. The point is not to advocate for any particular rule. In particular, I do not seek to answer moral or norms-based questions about what content Internet platforms should take down. I do, however, note the serious tensions between calls for platforms to remove horrific but First Amendment-protected extremist content – a category that probably includes the Christchurch shooter’s video – and calls for them to function as “public squares” by leaving up any speech the First Amendment permits. To lay out the issue, I draw on analysis developed at greater length in previous publications. This analysis concerns large user-facing platforms like Facebook and Google, and the word “platform” as used here refers to those large companies, not their smaller counterparts.