KONDO INSULATORS - ARE SIMPLE THEORIES GOOD ENOUGH

We briefly summarize the basic concepts behind the terms ''Kondo insul ator''. We point out the importance of including band structure effect s in calculating excitation spectra. A discussion of a mid-gap impurit y band induced by dopants leads to the suggestion of a low-temperature magnetic phase caused by departure from stoichiometry in samples of C eNiSn. This could provide an explanation for the 1/T1 is-proportional- to T3 behavior observed in NMR for this material.