  A common approach for the numerical simulation of wave propagation on a
spatially unbounded domain is to truncate the domain via an artificial
boundary, thus forming a finite computational domain with an outer boundary.
Absorbing boundary conditions must then be specified at the boundary such that
the resulting initial-boundary value problem is well posed and such that the
amount of spurious reflection is minimized. In this article, we review recent
results on the construction of absorbing boundary conditions in General
Relativity and their application to numerical relativity.
