  For thermostatted dissipative systems the Fluctuation Theorem gives an
analytical expression for the ratio of probabilities that the time averaged
entropy production in a finite system observed for a finite time, takes on a
specified value compared to the negative of that value. Hitherto it had been
thought that the presence of some thermostatting mechanism was an essential
component of any system which satisfies a Fluctuation Theorem. In the present
paper we show that a Fluctuation Theorem can be derived for purely Hamiltonian
systems, with or without applied dissipative fields.
