  We briefly review the calculations of quantum corrections related with the
exact NSVZ $\beta$-function in ${\cal N}=1$ supersymmetric theories, paying
especial attention to the scheme dependence of the results. It is explained,
how the NSVZ relation is obtained for the renormalization group functions
defined in terms of the bare coupling constant if a theory is regularized by
higher derivatives. Also we describe, how to construct a special
renormalization prescription which gives the NSVZ relation for the
renormalization group functions defined in terms of the renormalized coupling
constant exactly in all orders for Abelian supersymmetric theories, regularized
by higher derivatives. The scheme dependence of the NSVZ $\beta$-function (for
the renormalization group functions defined in terms of the renormalized
coupling constant) is discussed in the non-Abelian case. It is shown that in
this case the NSVZ $\beta$-function leads to a certain scheme-independent
equality.
