- Many terror-related groups use the Web as a convenient, anonymous communication infrastructure. This infrastructure enables exchange of information and propagation of ideas to active and potential terrorists. The Terrorist Detection System (TDS) is aimed at tracking down suspected terrorists by analyzing the content of information they access. In this chapter we present an advanced version of TDS (ATDS), where the detection algorithm was enhanced to improve the detection and reduce the false alarms. ATDS was implemented and evaluated in a network environment of 38 users comparing it to the performance of the basic TDS. Behavior of suspected terrorists was simulated by accessing known terror-related sites. The evaluation included also sensitivity analysis aimed at calibrating the settings of ATDS parameters to optimize its performance. The evaluation results suggest that ATDS outperformed TDS significantly and was able to reach very high detection rates when optimally tuned.