Terrorists have quickly learned to use the Internet as an accessible and cost-effective information infrastructure. Secure and non-secure web sites, online forums, and file-sharing services are routinely used by terrorist groups for spreading their propaganda, recruiting new members, communicating with their affiliates, and sharing knowledge on forgery, explosive preparation, and other "core" terrorist activities. The current number of known terrorist sites and active extremist forums is so large and their URL addresses are so volatile that a continuous manual monitoring of their multilingual content is definitely out of question. Moreover, terrorist web sites often try to conceal their real identity, e.g., by masquerading themselves as news portals or religious forums. This is why Web Intelligence and Web Mining methods are so important for efficiently securing the Web against its misuse by terrorists