Mat.Res.Soc.Symp.Proc.Vol.156, 349 (1989).
A fundamental asymmetry exists between electrons and holes in solids. Electrons at the Fermi surface give rise to high conductivity and normal metallic behavior, holes at the Fermi surface yield poor conductivity and give rise to superconductivity. We review here the theoretical basis for this assertion and its implications, particularly for the understanding of high temperature superconductivity in oxides.