  The description of nuclei starting from the constituent nucleons and the
realistic interactions among them has been a long-standing goal in nuclear
physics. In addition to the complex nature of the nuclear forces, with two-,
three- and possibly higher many-nucleon components, one faces the
quantum-mechanical many-nucleon problem governed by an interplay between bound
and continuum states. In recent years, significant progress has been made in ab
initio nuclear structure and reaction calculations based on input from
QCD-employing Hamiltonians constructed within chiral effective field theory.
After a brief overview of the field, we focus on ab initio many-body approaches
- built upon the No-Core Shell Model - that are capable of simultaneously
describing both bound and scattering nuclear states, and present results for
resonances in light nuclei, reactions important for astrophysics and fusion
research. In particular, we review recent calculations of resonances in the
$^6$He halo nucleus, of five- and six-nucleon scattering, and an investigation
of the role of chiral three-nucleon interactions in the structure of $^9$Be.
Further, we discuss applications to the $^7$Be$(p,\gamma)^8$B radiative
capture. Finally, we highlight our efforts to describe transfer reactions
including the $^3$H$(d,n)^4$He fusion.
