  The field of superconductivity in the class of materials known as graphite
intercalation compounds has a history dating back to the 1960s. This paper
recontextualizes the field in light of the discovery of superconductivity in
CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and
electronic structure of these and related compounds. We go on to experiments
addressing the superconducting energy gap, lattice dynamics, pressure
dependence, and how this relates to theoretical studies. The bulk of the
evidence strongly supports a BCS superconducting state. However, important
questions remain regarding which electronic states and phonon modes are most
important for superconductivity and whether current theoretical techniques can
fully describe the dependence of the superconducting transition temperature on
pressure and chemical composition.
