  A numerical-relativity calculation yields in general a solution of the
Einstein equations including also a radiative part, which is in practice
computed in a region of finite extent. Since gravitational radiation is
properly defined only at null infinity and in an appropriate coordinate system,
the accurate estimation of the emitted gravitational waves represents an old
and non-trivial problem in numerical relativity. A number of methods have been
developed over the years to "extract" the radiative part of the solution from a
numerical simulation and these include: quadrupole formulas, gauge-invariant
metric perturbations, Weyl scalars, and characteristic extraction. We review
and discuss each method, in terms of both its theoretical background as well as
its implementation. Finally, we provide a brief comparison of the various
methods in terms of their inherent advantages and disadvantages.
