  Some of the theoretical challenges posed by the general relativistic
description of binary systems of compact objects (neutron stars or black holes)
are reviewed. We recall the various ways one can use the theory of the motion,
and of the timing, of binary pulsars to test the strong-field and/or radiative
aspects of General Relativity. Recent advances in the theory of the motion and
radiation of binary black holes are discussed. One emphasizes the usefulness of
the Effective One Body approach in providing a quasi-analytical description of
the waveform emitted by coalescing binary black holes.
