Computer systems and networks are considered as a union of all resources, i.e. hardware, software and people (users, administrators and managers), essential for the realization of predicted tasks. The system dependability is defined as a generalization of performability and reliability, combining the notions of both these terms. The functional-reliability models are based on the observation that only a subset of the system resources is involved in a task execution and only inefficiencies of these resources may influence the correctness of task realization. The systems are working in a real environment, which is often hostile: it may be a source of threats, such as security intrusions, faulty or modified software and human errors. The set of analyzed events is a sum of hardware failures and malfunctions, software faults, human errors,, intrusions (intended and addressed threats), and viruses (unaddressed threats that are broadcast in the system and in its environment).