  The constraint-preserving approach, which aim is to provide consistent
boundary conditions for Numerical Relativity simulations, is discussed in
parallel with other recent developments. The case of the Z4 system is
considered, and constraint-preserving boundary conditions of the Sommerfeld
type are provided. A necessary condition for the stability of the proposed
boundary conditions is obtained, which amounts to the requirement of a
symmetric ordering of space derivatives. This requirement is numerically seen
to be also sufficient in the absence of corners and edges. Maximally
dissipative boundary conditions are also implemented. In this case, a less
restrictive stability condition is obtained, which is shown numerically to be
also sufficient even in the presence of corners and edges.
