  Recently there has been considerable interest in the Fluctuation Theorem
(FT). The FT shows how time reversible microscopic dynamics leads to
irreversible macroscopic behavior as the system size or observation time
increases. We show that the argument of the Evans-Searles FT, the dissipation
function, plays a central role in nonlinear response theory and derive the
Dissipation Theorem, giving exact relations for nonlinear response of classical
N-body systems. These expressions should be verifiable experimentally. When
linearized they reduce to the Green-Kubo expressions for linear response.
