Chaotic behaviour of physical systems is not new. A gaz for example in a chaotic system par excellence. The root of the word gaz in chaos. However, chaos does not only arise in many particle systems. For a long time it is known that systems with only a few degrees of freedom may be chaotic. About one hundred years ago, Henri Poincaré criticized the mechanical worldview of classical physics. Although he directed his attack against the bastion of Newtonian physics, celestial mechanics, until very recently he and his followers have not been heard. In the meantime, a drastic change in attitude seems to take place: Chaotic phenomena are not only detected in all established areas of natural science but also are taken for serious, and investigated. Of course, it
cannot yet be deckled whether this is a change in paradigm in the sense of T.S. Kuhn , although there are many hints for it. Especially, the change of the physical method which may be characterized by the slogan: it In. better to determine a reliable, general frame for the future systems behaviour than to make exact but incorrect quantitative predictions.