  A precise description of neutrino-nucleus reactions will play a key role in
addressing fundamental questions such as the leptonic CP violation and the
neutrino mass hierarchy through analyzing data from next-generation neutrino
oscillation experiments. The neutrino energy relevant to the neutrino-nucleus
reactions spans a broad range and, accordingly, the dominant reaction mechanism
varies across the energy region from quasi-elastic scattering through nucleon
resonance excitations to deep inelastic scattering. This corresponds to
transitions of the effective degree of freedom for theoretical description from
nucleons through meson-baryon to quarks. The main purpose of this review is to
report our recent efforts towards a unified description of the neutrino-nucleus
reactions over the wide energy range; recent overall progress in the field is
also sketched. Starting with an overview of the current status of
neutrino-nucleus scattering experiments, we formulate the cross section to be
commonly used for the reactions over all the energy regions. A description of
the neutrino-nucleon reactions follows and, in particular, a dynamical
coupled-channels model for meson productions in and beyond the $\Delta$(1232)
region is discussed in detail. We then discuss the neutrino-nucleus reactions,
putting emphasis on our theoretical approaches. We start the discussion with
electroweak processes in few-nucleon systems studied with the correlated
Gaussian method. Then we describe quasi-elastic scattering with nuclear
spectral functions, and meson productions with a $\Delta$-hole model. Nuclear
modifications of the parton distribution functions determined through a global
analysis are also discussed. Finally, we discuss issues to be addressed for
future developments.
