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.