  In this talk I will describe the deep influence Planck had on the development
of statistical mechanics. At this end I will first outline the theoretical
situation of statistical mechanics before Planck. I will then describe his main
contributions to this field and the progresses obtained as an immediate
consequence of his work. I will also outline the later evolution of statistical
mechanics in relation with Planck's work. I will finally report on a still
unsolved problem in statistical mechanics, historically related to the
properties of black-body radiation.
