In the last decade of the twentieth century, the rapid development of computer technology and its application in the military doctrine prompted the idea of network-centric warfare. Besides sea, air, land and space, the contemporary strategy defines cyberspace as the fifth battlefield and important part to conduct information operations. The purpose of this was to introduce a new revolution in military affairs, but also to demonstrate the superiority of the Western way of war. However, the opponent always adapts. Computers and internet are available to everyone, so the new paradigm did not become an exclusive right to the rich nations and powerful armies. Terrorist organizations around the world quickly recognized the usefulness of cyberspace, first to collect information, then for communication, planning and conducting terrorist actions. Especially the militant religious extremists from the beginning started to use cyberspace for propaganda, recruitment and training of new members, and trying to carry out cyber attacks. There is skepticism that the threat of terrorist cyber attacks is exaggerated, but the analysis of the most recent attempts shows that we have to understand the seriousness of their intention to cause severe damage on computer systems of critical infrastructure. In this context, the intention of this chapter is to show that the terrorists not only recognized cyberspace as the new battlefield, but there is a trend of their increased presence, suggesting the necessity to monitor their adaptation to new technologies and to estimate their capacities and capabilities, or to bring the war against terrorism into cyberspace as well.