We review theoretical aspects of the studies on
the recently discovered BiCh2 (Ch=chalcogen) based superconductors.
We first focus on the electronic band structure,
the Fermi surface and their correlation with the lattice
structure. We then discuss the phonon features and
the issue of the lattice instability. Finally, we survey the
phonon-mediated as well as the unconventional pairing
mechanisms that have been proposed so far.