  The general method of the reduction in the number of coupling parameters is
discussed. Using renormalization group invariance, theories with several
independent couplings are related to a set of theories with a single coupling
parameter. The reduced theories may have particular symmetries, or they may not
be related to any known symmetry. The method is more general than the
imposition of invariance properties. Usually, there are only a few reduced
theories with an asymptotic power series expansion corresponding to a
renormalizable Lagrangian. There also exist `general' solutions containing
non-integer powers and sometimes logarithmic factors. As an example for the use
of the reduction method, the dual magnetic theories associated with certain
supersymmetric gauge theories are discussed. They have a superpotential with a
Yukawa coupling parameter. This parameter is expressed as a function of the
gauge coupling. Given some standard conditions, a unique, isolated power series
solution of the reduction equations is obtained. After reparametrization, the
Yukawa coupling is proportional to the square of the gauge coupling parameter.
The coefficient is given explicitly in terms of the numbers of colors and
flavors. `General' solutions with non-integer powers are also discussed. A
brief list is given of other applications of the reduction method.
