  Suppose that D is a planar Jordan domain and x and y are distinct boundary
points of D. Fix \kappa \in (4,8) and let \eta\ be an SLE_\kappa process from x
to y in D. We prove that the law of the time-reversal of \eta is, up to
reparameterization, an SLE_\kappa process from y to x in D. More generally, we
prove that SLE_\kappa(\rho_1;\rho_2) processes are reversible if and only if
both \rho_i are at least \kappa/2-4, which is the critical threshold at or
below which such curves are boundary filling.
  Our result supplies the missing ingredient needed to show that for all \kappa
\in (4,8) the so-called conformal loop ensembles CLE_\kappa\ are canonically
defined, with almost surely continuous loops. It also provides an interesting
way to couple two Gaussian free fields (with different boundary conditions) so
that their difference is piecewise constant and the boundaries between the
constant regions are SLE_\kappa curves.
