  Minkowski space is shown to be globally stable as a solution to the
Einstein--Vlasov system in the case when all particles have zero mass. The
proof proceeds by showing that the matter must be supported in the "wave zone",
and then proving a small data semi-global existence result for the
characteristic initial value problem for the massless Einstein--Vlasov system
in this region. This relies on weighted estimates for the solution which, for
the Vlasov part, are obtained by introducing the Sasaki metric on the mass
shell and estimating Jacobi fields with respect to this metric by geometric
quantities on the spacetime. The stability of Minkowski space result for the
vacuum Einstein equations is then appealed to for the remaining regions.
