At high temperatures the localized f-electrons form independent local magnetic moments. According to the Kondo effect, the dc-resistivity of Kondo insulators shows a logarithmic temperature-dependence. At low temperatures, the local magnetic moments are screened by the sea of conduction electrons, forming a so-called Kondo resonance. The interaction of the conduction band with the f-orbitals results in a hybridization and an energy gap ϵ ϵ g . If the chemical potential lies in the hybridization gap, an insulating behavior can be seen in the dc-resistivity at low temperatures.