Among many f-electrons (rare-earth and actinides) compounds especially interesting are so called “heavy fermion” (HF) systems. Anomalously high values of electronic specific heat and low-temperature magnetic susceptibility evidence high effective mass m* value for charge carriers that can be tens and hundreds time larger than electron mass m. Magnetic and transport properties of such compounds are strongly temperature dependent. For high T f-electrons seems to be weakly interacting with itinerant electrons forming localized magnetic moments. However for low temperatures the picture is drastically different: local magnetic moments are suppressed and transport properties correspond to approaching to insulating state with T decrease. At T = 0 they could become insulators due to gap appearance on Fermi surface. Such systems are called Kondo insulators.