Threats are not protected speech, but defining what constitutes a threat has been problematic, particularly when it comes to online speech. We start with a look at threats against the US president, beginning with a 1798 prosecution for threatening John Adams, and leading up to the passage in 1917 of the first federal legislation against threatening the president. We will look at WW I-era prosecutions for threatening the president, leading up to a 1969 Supreme Court decision, Watts v. US, distinguishing true threats from protected political speech. And we conclude with two cases of online threats: the prosecution of Anthony Elonis for posting threats on Facebook, and a case where two tourists were denied entry to the US because of joking tweets that were treated as threats by US Border Agents. We conclude that threats, like obscenity, remain unprotected speech, but defining what is and what is not a threat in any particular case remains a problematic, subjective decision.