F.Bonetto, G.Gallavotti, A.Giuliani, F.Zamponi Chaotic Hypothesis, Fluctuation Theorem and singularities (72K, Revtex4 with 1 eps figure) ABSTRACT. Recently the ``Fluctuation theorem'' has been criticized and incorrect contents have been attributed to it. Here we reestablish and comment the original statements. We also discuss aspects of the extension of the chaotic hypothesis and of the fluctuation relation to singular systems.