The success of the 1967 Battelle Rencontres was so much appre­ ciated by the participants and organizers of this experimental set-up that it was soon decided to go on with the experiment. Mathematicians and physicists had found a very suitable frame to overcome their natural shyness, to get occasionally interested into each others' work, to talk 1968 Rencontres have about it, and eventually to know each other. The been organized with the same idea in mind, and even somewhat enlarged in the following sense: the topic chosen - hyperbolic equations and waves - has proved a cornerstone of physics for more than a century and extends over most fields of contemporary physics. It follows immediately that the wide range of physicists concerned could not be represented by more than a couple of specialists in any single field. Thus, aside from bridging the gap between mathematicians and physicists, the 1968 Recontres provided a rather unique occasion to plug many intra­ disciplinary gaps among physicists. This made the Rencontres quite unpredictable as to how people would - and could - interact, and created a very stimulating environ­ ment for an unprecedented intellectual venture. From the outside, it may very well look like a hodge-podge of quite unrelated ideas. But it was much less so at the level of day-to-day discussions and informal gatherings where all slowly acquired a comprehensive synthetic view of the subject.