  FeSb$_2$ has been recently identified as a new model system for studying
many-body renormalizations in a $d$-electron based narrow gap semiconducting
system, strongly resembling FeSi. The electron-electron correlations in
FeSb$_2$ manifest themselves in a wide variety of physical properties including
electrical and thermal transport, optical conductivity, magnetic
susceptibility, specific heat and so on. We review some of the properties that
form a set of experimental evidences revealing the crucial role of correlation
effects in FeSb$_2$. The metallic state derived from slight Te doping in
FeSb$_2$, which has large quasiparticle mass, will also be introduced.
