  The present understanding of reaction processes involving light unstable
nuclei at energies around the Coulomb barrier is reviewed. The effect of
coupling to direct reaction channels on elastic scattering and fusion is
investigated, with the focus on halo nuclei. A list of definitions of processes
is given, followed by a review of the experimental and theoretical tools and
information presently available. The effect of couplings on elastic scattering
and fusion is studied with a series of model calculations within the
coupled-channels framework. The experimental data on fusion are compared to
"bare" no-coupling one-dimensional barrier penetration model calculations. On
the basis of these calculations and comparisons with experimental data,
conclusions are drawn from the observation of recurring features. The total
fusion cross sections for halo nuclei show a suppression with respect to the
"bare" calculations at energies just above the barrier that is probably due to
single neutron transfer reactions. The data for total fusion are also
consistent with a possible sub-barrier enhancement; however, this observation
is not conclusive and other couplings besides the single-neutron channels would
be needed in order to explain any actual enhancement. We find that a
characteristic feature of halo nuclei is the dominance of direct reactions over
fusion at near and sub-barrier energies; the main part of the cross section is
related to neutron transfers, while calculations indicate only a modest
contribution from the breakup process.
