  The giant molecular clouds (MCs) found in the Milky Way and similar galaxies
play a crucial role in the evolution of these systems. The supernova explosions
that mark the death of massive stars in these regions often lead to
interactions between the supernova remnants (SNRs) and the clouds. These
interactions have a profound effect on our understanding of SNRs. Shocks in
SNRs should be capable of accelerating particles to cosmic ray (CR) energies
with efficiencies high enough to power Galactic CRs. X-ray and gamma-ray
studies have established the presence of relativistic electrons and protons is
some SNRs and provided strong evidence for diffusive shock acceleration as the
primary acceleration mechanism, including strongly amplified magnetic fields,
temperature and ionization effects on the shock-heated plasmas, and
modifications to the dynamical evolution of some systems. Because protons
dominate the overall energetics of the CRs, it is crucial to understand this
hadronic component even though electrons are much more efficient radiators and
it can be difficult to identify the hadronic component. However, near MCs the
densities are sufficiently high to allow the gamma-ray emission to be dominated
by protons. Thus, these interaction sites provide some of our best
opportunities to constrain the overall energetics of these particle
accelerators. Here we summarize some key properties of interactions between
SNRs and MCs, with an emphasis on recent X-ray and gamma-ray studies that are
providing important constraints on our understanding of cosmic rays in our
Galaxy.
