The growing use of mathematical models in hydraulics has not made physical models obsolete. They keep pace with mathematical models and in some cases make progress in conjunction with them Physical models continue to be developed: more precise use of similitude criteria, better knowledge of scale effects, new and more complex types of model, use of various artifices, more ade quate instrumentation, automation of the operation of model- all these pOint to that development. What was needed was to make a survey of the situation, and this was the principal aim of the NATO Advanced Study Institute...