In all societies, the normal flow of life is occasionally interrupted by critical episodes, accompanied by a sense of threat and insecurity, challenging the way in which people understand the world around them. Such situations, called crises, can be caused by natural forces (earthquakes, tsunamis, storm winds, torrential rain, snowstorms, avalanches, epidemics), intentional actions of "others" (ie: various enemies inside or outside the society) -- international conflicts and war, terrorist attacks -- and human errors in managing technology, to name a few. But their roots may also be in poorly functioning socio-technical and administrative systems (infrastructure breakdowns, industrial accidents, economic crises, and political scandals). Historically, we can say that crises are increasing in number and becoming more diverse by nature. In the early development of civilisation, the main threat came from nature and conflicts between human groups, whereas in modern post-industrial societies, these are joined by risks coming from engineering and technology. In addition to old crises whose causes, consequences and time curves are more or less known, modern crises also appear, against which the human mind is powerless, organisational capacities are insufficient, and which make the public existentially concerned.