Risk has become a key concept in modern society. Growing concern about the environment and a number of disasters have served to focus attention on the hazards and risks involved in a wide range of activities from offshore oil production to rail and air transport; from the design of football stadium to the operation of chemical plants and environmental protection. Today there is a wide range of techniques available to assess risk and reliability, both in relation to safety and in the wider sense. These techniques now underpin new legislation on safety and have relevance over a broad spectrum of activities, including environmental and other systems, where risk and reliability are key concerns. The concept of a complex system applies not only to the technical ones but also the infrastructure of major importance for social life such as transportation and logistics systems, buildings, power systems, water distribution systems or health services.