  The theory of hole superconductivity proposes that there is a single
mechanism of superconductivity that applies to all superconducting materials.
This paper discusses several material families where superconductivity occurs
and how they can be understood within this theory. Materials discussed include
the elements, transition metal alloys, high $T_c$ cuprates both hole-doped and
electron-doped, $MgB_2$, iron pnictides, doped semiconductors, and elements
under high pressure.
