  The idea of reduction of couplings in renormalizable theories will be
presented and then will be applied in Particle Physics models. Reduced
couplings appeared as functions of a primary one, compatible with the
renormalization group equation and thus solutions of a specific set of ordinary
differential equations. If these functions have the form of power series the
respective theories resemble standard renormalizable ones and thus widen
considerably the area covered until then by symmetries as a tool for
constraining the number of couplings consistently. Still on the more abstract
level reducing couplings enabled one to construct theories with beta-functions
vanishing to all orders of perturbation theory. Reduction of couplings became
physics-wise truly interesting and phenomenologically important when applied to
the standard model and its possible extensions. In particular in the context of
supersymmetric theories it became the most powerful tool known today once it
was learned how to apply it also to couplings having dimension of mass and to
mass parameters. Technically this all relies on the basic property that
reducing couplings is a renormalization scheme independent procedure.
Predictions of top and Higgs mass prior to their experimental finding highlight
the fundamental physical significance of this notion.
