  In the last decades the theory of large deviations has become a main tool in
statistical mechanics especially in the study of non--equilibrium. In a
rational reconstruction of the story one must recognize the ideal connection
and debt of some recent work, to discussions taking place at the beginning of
the twentieth century. The famous equation $S=k\ln W$ usually attributed to
Boltzmann, actually written in this final form by Planck on his route to the
quantum hypothesis, was interpreted by Einstein as a large deviation formula.
This interpretation, on which he based his theory of thermodynamic equilibrium
fluctuations, has been a source of inspiration in recent developments of
non--equilibrium statistical mechanics. In this paper we briefly illustrate
this aspect.
