Kondo effect refers to the scattering of conduction electrons due to localized spins in a metal. When it happens in a dense local moment material, e.g. rare- earth intermetallic compounds, the strong electron correlation leads to a prominent phenomenon in condensed matter physics - the formation of heavy fermion or Kondo insulator, depending on the band filing. One of the most interesting questions of Kondo physics is the applicability of Kondo insulator framework to materials with only 3d electrons. This book will provide a study of an unconventional narrow band semiconductor FeSb2. We show that it is closely related to iron silicide FeSi, which is the first example of 3d-electron containing Kondo insulator and it exhibits many interesting properties, including giant carrier mobility, multiband transport, giant thermoelectric power factor, and colossal magnetoresistance with chemical substitution. It is a reference point of understanding this material and will be useful to students and researchers in the field.