The development of the Internet has revolutionised communications. It has never been easier to speak to wide audiences or to communicate with people that may be located more than half a world away from the speaker. However, like any neutral platform, the Internet can be used to many different ends, including illegal, offensive, or dangerous purposes. Terrorist groups, such as the Islamic State (IS, also referred to as ISIS or ISIL), Al Qaeda, Hamas, and Al Shabaab, use the Internet to disseminate their ideology, to recruit new members, and to take credit for attacks around the world. Several U.S. policymakers, including some Members of Congress, have expressed concern about the influence that terrorist advocacy may have upon those who view or read it. The ease with which such speech may be disseminated over the Internet, using popular social media services, has been highlighted by some observers as potentially increasing the ease by which persons who might otherwise have not been exposed to the ideology or recruitment efforts of terrorist entities may become radicalised. These concerns raise the question of whether it would be permissible for the federal government to restrict or prohibit the publication and distribution of speech that advocates the commission of terrorist acts when that speech appears on the Internet. This book discusses relevant precedent concerning the extent to which advocacy of terrorism may be restricted in a manner consistent with the First Amendment’s Freedom of Speech Clause. The book also discusses the potential application of the federal ban on the provision of material support to foreign terrorist organisations (FTOs) to the advocacy of terrorism, including as it relates to the dissemination of such advocacy via online services like Twitter or Facebook.