3d heavy fermions: Do they exist?

Dilute intermetallic compounds of 3d elements are compared with those of 4f and 5f elements showing Kondo or intermediate valence character. New results on Co-Zn intermetallics are presented which show that anomalous low temperature behaviour of susceptibility and specific heat can be found even when the latter is very small because of nearly full Brillouin zone character. A distinction is drawn between Kondo insulators and hybridization insulators, with special reference to FeSi and RuAl(2).