This article assesses the cases that have come to light since 9/11 of Islamist extremist terrorism, whether based in the United States or abroad, in which the United States itself has been, or apparently has been, targeted. Information from them is used to evaluate how the Internet (including various forms of electronic communication) has affected several aspects of the terrorism enterprise in the United States: radicalization, communication, organization, and the gathering of information. In general, it is found that the Internet has not been particularly important. Although it has been facilitating in some respects, it has scarcely ever been necessary. In some respects, the Internet more fully aids efforts to police terrorism – although this is mainly due to the incompetence and amateurishness of would-be terrorists. In other respects, however, the Internet, and the big data compilations it makes possible, greatly increase the costs and complications of the counterterrorism quest.