  We prove exponential decay of correlations for the billiard flow associated
with a two-dimensional finite horizon Lorentz Gas (i.e., the Sinai billiard
flow with finite horizon). Along the way, we describe the spectrum of the
generator of the corresponding semi-group L_t of transfer operators, i.e., the
resonances of the Sinai billiard flow, on a suitable Banach space of
anisotropic distributions. (Revised after referees' comments.)
