This morning session is devoted to problems of statistical physics, and I hope that together with my colleagues Dr Kac and Dr Cohen we will be able to show you that in this field very little is really understood, and that there remain quite general and fundamental problems which are still quite open. Since in fact even the formulation of these problems is often still controversial, let me begin with what I consider to be the basic task of statistical mechanics. It is, in my opinion, the elucidation of the relation between the microscopic, molecular description and the macroscopic description of the physical phenomena. This is illustrated in Fig. 1, where the microscopic description is divided according to wherther the classical or the more basic quantum theory is considered, and where only some of the macroscopic disciplines are shown. Note that I said relation and not explanation. I consider the two descriptions of nature as autonomous.