A widespread interest has arisen in recent years in intermetallic compounds which show a variety of unusual properties. Prominent amongst these is a very large electronic contribution to the specific heat at low temperatures which can be parametrized in terms of an effective mass hundreds or thousands of times larger than those in normal metals. The background to this behaviour is outlined and some of the electrical, magnetic and superconducting properties of such materials, mainly alloys of Ce or U, are presented.