Terrorism is by no means a new phenomenon. In recent decades, however, it has become a major security threat that requires a shift in the focus of the defence efforts of many developed countries to designing effective anti-terrorism strategies, and corresponding R&D strategies. This Chapter analyses the main characteristics of terrorist organisations with a view to identifying their main weaknesses. We find that the key weaknesses of terrorist organisations are their small size, their high vulnerability to any action against their leadership, and their high susceptibility to the use of high-tech intelligence gathering and processing systems. The Chapter recommends that the main anti-terrorism strategies should be active (pre-emptive) and focus on applying continuous pressure on key activists (including leaders) of terrorist organisations. Since such pressure is attainable only through the coordinated effort of a broad range of defence-related agencies, the emphasis of the anti-terrorism R&D strategy should be on investment in integrative technologies and in particular on long-term intelligence (LTI) systems.