  We present a review of the spin Hurwitz numbers, which count the ramified
coverings with spin structures. They are related to peculiar $Q$ Schur
functions, which are actually related to characters of the Sergeev group. This
allows one to put the whole story into the modern context of matrix models and
integrable hierarchies. Hurwitz partition functions are actually broader than
conventional $\tau$-functions, but reduce to them in particular circumstances.
We explain, how a special $d$-soliton $\tau$-functions of KdV and
Veselov-Novikov hierarchies generate the spin Hurwitz numbers $H^\pm\left(
\Gamma^r_d \right)$ and $H^\pm\left( \Gamma^r_d,\Delta \right)$. The generating
functions of the spin Hurwitz numbers are hypergeometric $\tau$-functions of
the BKP integrable hierarchy, and we present their fermionic realization. We
also explain how one can construct $\tau$-functions of this type entirely in
terms of the $Q$ Schur functions. An important role in this approach is played
by factorization formulas for the $Q$ Schur functions on special loci.
