  We formulate the rules for dimensional reduction of a generic finite
temperature gauge theory to a simpler three-dimensional effective bosonic
theory in terms of a matching of Green's functions in the full and the
effective theory, and present a computation of a generic set of 1- and 2-loop
graphs needed for the application of these rules. As a concrete application we
determine the explicit mapping of the physical parameters of the standard
electroweak theory to a three-dimensional SU(2)xU(1) gauge-Higgs theory. We
argue that this three-dimensional theory has a universal character and appears
as an effective theory for many extensions of the Standard Model.
