The monograph is concerned with the modulus of families of curves on
Riemann surfaces and its applications to extremal problems for
conformal, quasiconformal mappings, and the extension of the modulus
onto Teichmüller spaces. The main part of the monograph deals with
extremal problems for compact classes of univalent conformal and
quasiconformal mappings. Many of them are grouped around two-point
distortion theorems. Montel's functions and functions with fixed angular
derivatives are also considered. The last portion of problems is
directed to the extension of the modulus varying the complex structure
of the underlying Riemann surface that sheds some new light on the
metric problems of Teichmüller spaces.