The paper addresses specific features of assessing terrorist risks for complex technical systems (CTS). These include feedback between CTS vulnerability towards a specific type of terrorist attack and the threat of such attack, ability of terrorists to learn lessons from previous attacks, react upon actions taken by counterterrorist forces; high level of uncertainty regarding terrorists' intentions, resources, and system of values. Conventional safety analysis for CTS is to be focused on the question: What is the way for an accident scenario to be realized in the given system? When addressing security problems for CTS one should also consider the situation from the terrorist's standpoint. Hence the modified question for security analysis should be: What is to be done for the given scenario to be realized in CTS? Two types of attacks at complex technical systems are assessed: (1) Attack of technological terrorism implies powerful unauthorized impacts at CTS capable of: (a) breaking through the CTS protection system; (b) initiating secondary catastrophic processes due to hazardous substances, energy, and information, stored or processed at the CTS; (c) escalation of the accident outside the CTS boundaries with substantially increased secondary and cascade losses. (2) Attack of intelligent terrorism (smart terrorism, insiders terrorism), i.e. a purposeful unauthorized interference into the process of designing, building and/or operating the CTS aimed at the increase of its existing vulnerabilities and creation of new ones in the system so that to use these input vulnerabilities, insider's knowledge of the system and access to its elements for future realization of most disastrous scenarios of a terrorist attack. Comparative assessment of these two types of terrorism is presented. Dynamic three-sided models that allow one to assess the situation from standpoints of terrorists, law enforcement agencies and administrations of CTS and analyze actions and counteractions of various sides involved.