  This chapter describes topological (Dirac and Weyl) semimetals from the
viewpoint of their observable electromagnetic response. We argue that this
response may be represented by topological terms with unquantized (non-integer)
coefficients and make a connection with the Luttinger's theorem, which relates
the size of the Fermi surface of an ordinary metal to the number of electrons
per unit cell. We discuss observable transport phenomena, associated with this
topological response.
