  The exact treatment of nuclei starting from the constituent nucleons and the
fundamental interactions among them has been a long-standing goal in nuclear
physics. Above all nuclear scattering and reactions, which require the solution
of the many-body quantum-mechanical problem in the continuum, represent an
extraordinary theoretical as well as computational challenge for {\em ab
initio} approaches. We present a new {\em ab initio} many-body approach which
derives from the combination of the {\em ab initio} no-core shell model with
the resonating-group method [4]. By complementing a microscopic cluster
technique with the use of realistic interactions, and a microscopic and
consistent description of the nucleon clusters, this approach is capable of
describing simultaneously both bound and scattering states in light nuclei. We
will discuss applications to neutron and proton scattering on $s$- and light
$p$-shell nuclei using realistic nucleon-nucleon potentials, and outline the
progress toward the treatment of more complex reactions.
