Trends in superconductivity of heavy-electron metals

Superconductivity in metals with electrons exhibiting a strongly enhanced e ffective mass may still be considered as an exotic phenomenon. In various c ases, convincing evidence for unconventional superconducting states has bee n found and it seems quite conceivable that superconductivity in these mate rials is not triggered by the usual electron-phonon interaction. We briefly review the present state of knowledge and also discuss a few recent develo pments.