Topological semimetals are a new class of materials which may be regarded as three-dimensional analogs of graphene. Their band structures present linear dispersions in the three directions of momentum, with crossings at Dirac or Weyl points. These topological semimetals exhibit extremely large carrier mobilities and low carrier densities. In this presentation I will discuss diverse types of topological semimetal materials through optical and magneto-optical spectroscopy combined with theoretical approach. These results provide indirect proof of Dirac/Weyl fermions' presence linked with characteristic optical and magneto-optical response of their band structure.